app: Add-program flow with server-side filesystem picker

Register an existing repo as a program from the /programs page — the web
equivalent of `castle program add`. An inline form with an editable,
autocompleting path bar browses the *server's* filesystem (a browser's own
file dialog only sees the client machine) and also accepts a git URL.

- core: extract the adopt logic (target parsing, stack/command detection,
  ProgramSpec build) into castle_core.adopt so the CLI and API share it; the
  CLI's add.py now delegates to it.
- castle-api: GET /fs/browse (directory listing, per-entry stat errors skipped
  so one unreadable child can't 403 the dir) and POST /programs/adopt.
- app: AddProgramForm + Add-program button on the Programs page; useBrowse /
  useAdoptProgram hooks.

Also tidy ProgramCard: deployments render below the description, and the
deployment name label is hidden when it matches the program title.

core/src/castle_core/adopt.py

@@ -0,0 +1,157 @@
+"""Adopt an existing repo as a program — shared by the CLI (`castle program add`)
+and the API (`POST /programs/adopt`).
+
+`create` scaffolds new code from a stack; `add` adopts code that already exists —
+a local path, or a git URL to clone later. Keeping the target parsing, stack /
+command sniffing, and ProgramSpec build here means both front-ends behave
+identically (the logic used to live only in the CLI).
+"""
+
+from __future__ import annotations
+
+import tomllib
+from dataclasses import dataclass, field
+from pathlib import Path
+
+from castle_core.config import CastleConfig
+from castle_core.manifest import BuildSpec, CommandsSpec, ProgramSpec
+
+
+class AdoptError(ValueError):
+    """A program can't be adopted (bad path, or the name already exists)."""
+
+
+def is_git_url(s: str) -> bool:
+    return s.startswith(("http://", "https://", "git@", "ssh://")) or s.endswith(".git")
+
+
+def looks_like_program(src: Path) -> bool:
+    """Whether a directory holds something castle can adopt (a project manifest or
+    a git repo). Used to flag candidates in the filesystem browser."""
+    return (
+        (src / ".git").exists()
+        or (src / "pyproject.toml").exists()
+        or (src / "Cargo.toml").exists()
+        or (src / "package.json").exists()
+        or (src / "Makefile").exists()
+        or (src / "makefile").exists()
+    )
+
+
+def detect_stack_commands(src: Path) -> tuple[str | None, dict[str, list[list[str]]]]:
+    """Detect (stack, commands) for a source dir.
+
+    Returns a stack name when the project fits a known one (so it inherits those
+    defaults), otherwise an explicit commands map. Adoption takes source only; the
+    deployment (and thus kind) is declared separately, so no kind is inferred here.
+    """
+    commands: dict[str, list[list[str]]] = {}
+
+    pyproject = src / "pyproject.toml"
+    if pyproject.exists():
+        try:
+            data = tomllib.loads(pyproject.read_text())
+        except (OSError, tomllib.TOMLDecodeError):
+            data = {}
+        deps = " ".join(data.get("project", {}).get("dependencies", []))
+        stack = "python-fastapi" if ("fastapi" in deps or "uvicorn" in deps) else "python-cli"
+        return stack, commands
+
+    if (src / "Cargo.toml").exists():
+        commands = {
+            "build": [["cargo", "build", "--release"]],
+            "test": [["cargo", "test"]],
+            "lint": [["cargo", "clippy"]],
+            "run": [["cargo", "run"]],
+        }
+        return None, commands
+
+    if (src / "package.json").exists():
+        commands = {
+            "build": [["pnpm", "build"]],
+            "test": [["pnpm", "test"]],
+            "lint": [["pnpm", "lint"]],
+        }
+        return None, commands
+
+    if (src / "Makefile").exists() or (src / "makefile").exists():
+        commands = {"build": [["make"]], "test": [["make", "test"]]}
+        return None, commands
+
+    return None, commands
+
+
+@dataclass
+class Adopted:
+    """The result of building a program spec from an adopt target — the caller
+    persists it (``config.programs[name] = spec``; then save/write)."""
+
+    name: str
+    spec: ProgramSpec
+    source: str
+    stack: str | None = None
+    repo: str | None = None
+    commands: list[str] = field(default_factory=list)
+
+
+def build_adopted_program(
+    config: CastleConfig,
+    target: str,
+    name: str | None = None,
+    description: str = "",
+) -> Adopted:
+    """Build (but do not save) a ProgramSpec adopting ``target``.
+
+    ``target`` is a local path or a git URL. Raises :class:`AdoptError` if a local
+    path doesn't exist or the resolved name already exists in the config. Does not
+    mutate ``config`` — the caller assigns ``config.programs[name]`` and persists.
+    """
+    repo_url: str | None = None
+
+    if is_git_url(target):
+        repo_url = target
+        name = name or Path(target.rstrip("/")).name.removesuffix(".git")
+        # Default local clone location; cloned later via `castle clone`.
+        source = str(config.repos_dir / name)
+        src_path = Path(source)
+    else:
+        src_path = Path(target).expanduser().resolve()
+        if not src_path.exists():
+            raise AdoptError(f"path does not exist: {src_path}")
+        source = str(src_path)
+        name = name or src_path.name
+
+    if name in config.programs or config.deployments_named(name):
+        raise AdoptError(f"'{name}' already exists in castle.yaml")
+
+    # Detect verbs from the working copy if we have one on disk. `kind` is derived
+    # from a deployment, not stored on the program — so adoption takes source only;
+    # declare a deployment separately (castle service/job create).
+    stack: str | None = None
+    detected: dict[str, list[list[str]]] = {}
+    if src_path.exists():
+        stack, detected = detect_stack_commands(src_path)
+
+    spec = ProgramSpec(
+        id=name,
+        description=description or f"Adopted from {target}",
+        source=source,
+        stack=stack,
+        repo=repo_url,
+    )
+    # `build` is declared via BuildSpec; every other verb via CommandsSpec.
+    if detected:
+        build_cmds = detected.pop("build", None)
+        if build_cmds:
+            spec.build = BuildSpec(commands=build_cmds)
+        if detected:
+            spec.commands = CommandsSpec.model_validate(detected)
+
+    return Adopted(
+        name=name,
+        spec=spec,
+        source=source,
+        stack=stack,
+        repo=repo_url,
+        commands=sorted(detected),
+    )