Add stack dependency management

Stacks declare host toolchains (uv/pnpm/node/hugo/deno/psql) that can drift
from what's installed and, worse, from what's on a running service's PATH.
Make those dependencies first-class and visible.

Model (core):
- stacks.ToolRequirement + StackHandler.tools declare each stack's toolchains
  (command, purpose, phase, install_hint); tools_for() is the single source.
- relations synthesizes them as `kind: tool` requirements so functional?/graph
  account for them; checked runtime-env-aware (run-phase tools of a systemd
  service are probed against the service's PATH, not the shell) via a shared
  generators.systemd.runtime_path helper the unit generator also uses, so the
  checker can't drift from the generator. hint_for() makes every unmet
  requirement actionable.
- stack_status: the derived per-stack health the CLI/API/UI all render.
- config: add ~/.deno/bin to USER_TOOL_PATH_DIRS so deno (supabase edge fns)
  is found by services and the check, same as ~/.local/bin and the pnpm dirs.

Surfaces:
- castle stack list|info (new resource) + GET /stacks/status, /stacks/{name}
  (GET /stacks stays a bare name list for the create-form select).
- castle doctor gains a "Stacks & dependencies" section (FAIL for an enabled
  deployment's missing tool, WARN otherwise, unused stacks skipped).
- castle apply preflight warns (advisory, like _acme_preflight) when a tool is
  missing where a service runs; ConvergePanel renders those warnings.
- Dashboard Stacks page: per-stack tool checklist with versions + copyable
  install hints, program links, and verb chips.

Tests: relations (drift + hints), doctor (ok/fail/skip), /stacks endpoints.

app/src/pages/Stacks.tsx

@@ -0,0 +1,158 @@
+import { useState } from "react"
+import { Link } from "react-router-dom"
+import { Check, Copy, Layers, X } from "lucide-react"
+import { cn } from "@/lib/utils"
+import { useStacksStatus, type StackStatus, type ToolStatus } from "@/services/api/hooks"
+import { PageHeader } from "@/components/PageHeader"
+
+// A stack's overall health pill: green when every tool it needs is present, red
+// when one is missing, grey when nothing on this node uses it yet.
+function StackBadge({ stack }: { stack: StackStatus }) {
+  const [label, cls] = !stack.in_use
+    ? (["unused", "bg-gray-700/50 text-gray-400"] as const)
+    : stack.ok
+      ? (["ready", "bg-green-800/50 text-green-300"] as const)
+      : (["missing tools", "bg-red-800/50 text-red-300"] as const)
+  return (
+    <span className={cn("inline-flex items-center text-xs px-2 py-0.5 rounded-full", cls)}>
+      {label}
+    </span>
+  )
+}
+
+// A copy-to-clipboard chip for a tool's install command — the "diagnose + copyable
+// hint" contract: we never install for you, but the fix is one click from your shell.
+function CopyHint({ text }: { text: string }) {
+  const [copied, setCopied] = useState(false)
+  return (
+    <button
+      onClick={() => {
+        void navigator.clipboard?.writeText(text)
+        setCopied(true)
+        window.setTimeout(() => setCopied(false), 1200)
+      }}
+      className="group inline-flex items-center gap-1.5 rounded border border-[var(--border)] bg-black/20 px-2 py-1 font-mono text-xs text-[var(--muted)] hover:border-[var(--primary)] hover:text-[var(--foreground)] transition-colors"
+      title="Copy install command"
+    >
+      {copied ? <Check size={12} className="text-green-400" /> : <Copy size={12} />}
+      <span className="truncate">{text}</span>
+    </button>
+  )
+}
+
+function ToolRow({ tool }: { tool: ToolStatus }) {
+  return (
+    <div className="flex flex-col gap-1 py-1.5">
+      <div className="flex items-center gap-2 text-sm">
+        {tool.present ? (
+          <Check size={14} className="shrink-0 text-green-400" />
+        ) : (
+          <X size={14} className="shrink-0 text-red-400" />
+        )}
+        <span className="font-mono font-medium">{tool.command}</span>
+        <span className="text-xs text-[var(--muted)]">
+          {tool.purpose} · {tool.phase}
+        </span>
+        {tool.version && (
+          <span className="ml-auto truncate max-w-[45%] text-xs text-[var(--muted)]" title={tool.version}>
+            {tool.version}
+          </span>
+        )}
+      </div>
+      {!tool.present && (
+        <div className="pl-6">
+          <CopyHint text={tool.install_hint} />
+        </div>
+      )}
+    </div>
+  )
+}
+
+function StackCard({ stack }: { stack: StackStatus }) {
+  return (
+    <div
+      className={cn(
+        "rounded-lg border bg-[var(--card)] p-4",
+        stack.in_use ? "border-[var(--border)]" : "border-[var(--border)] opacity-60",
+      )}
+    >
+      <div className="flex items-center justify-between gap-2">
+        <h2 className="font-semibold">{stack.name}</h2>
+        <StackBadge stack={stack} />
+      </div>
+
+      <div className="mt-3 divide-y divide-[var(--border)]">
+        {stack.tools.length > 0 ? (
+          stack.tools.map((t) => <ToolRow key={t.command} tool={t} />)
+        ) : (
+          <p className="py-1.5 text-sm text-[var(--muted)]">No host tools required.</p>
+        )}
+      </div>
+
+      {stack.programs.length > 0 && (
+        <div className="mt-3 flex flex-wrap items-center gap-1.5 text-xs">
+          <span className="text-[var(--muted)]">used by</span>
+          {stack.programs.map((p) => (
+            <Link
+              key={p}
+              to={`/programs/${p}`}
+              className="rounded bg-black/20 px-1.5 py-0.5 font-mono hover:text-[var(--primary)]"
+            >
+              {p}
+            </Link>
+          ))}
+        </div>
+      )}
+
+      {stack.verbs.length > 0 && (
+        <div className="mt-2 flex flex-wrap gap-1.5 text-[11px] text-[var(--muted)]">
+          {stack.verbs.map((v) => (
+            <span key={v} className="rounded-full border border-[var(--border)] px-1.5 py-0.5">
+              {v}
+            </span>
+          ))}
+        </div>
+      )}
+    </div>
+  )
+}
+
+export function Stacks() {
+  const { data: stacks, isLoading } = useStacksStatus()
+  const missing = (stacks ?? []).filter((s) => s.in_use && !s.ok).length
+
+  return (
+    <div className="max-w-6xl mx-auto px-6 py-8">
+      <PageHeader
+        title="Stacks"
+        subtitle="The toolchains each stack needs, and whether they're present where your services run"
+        actions={
+          missing > 0 ? (
+            <span className="inline-flex items-center gap-1.5 text-sm text-red-400">
+              <X size={14} /> {missing} stack{missing !== 1 ? "s" : ""} missing tools
+            </span>
+          ) : stacks && stacks.length > 0 ? (
+            <span className="inline-flex items-center gap-1.5 text-sm text-green-400">
+              <Check size={14} /> all toolchains present
+            </span>
+          ) : null
+        }
+      />
+
+      {isLoading ? (
+        <p className="text-[var(--muted)]">Loading...</p>
+      ) : stacks && stacks.length > 0 ? (
+        <div className="grid gap-4 sm:grid-cols-2">
+          {stacks.map((s) => (
+            <StackCard key={s.name} stack={s} />
+          ))}
+        </div>
+      ) : (
+        <div className="flex flex-col items-center gap-2 py-16 text-[var(--muted)]">
+          <Layers size={28} />
+          <p>No stacks.</p>
+        </div>
+      )}
+    </div>
+  )
+}