refactor(prime-agent): extract the /mcpctl switcher to typechecked source; pi --dry-run; docs

The prime-agent switcher existed only as a 275-line string literal inside
prime-agent-extension.ts, so nothing typechecked or linted it — the exact gap
that let a wrong ctx.ui.select() option shape ship in the pi extension. It now
lives at src/prime-agent-ext/mcpctl-switch.ts with a generator, a tsconfig
checking it against the real @earendil-works/pi-coding-agent types, eslint
coverage and an embed-freshness test, matching pi and opencode.

The extraction was verified byte-identical before any edit, so the behaviour
shipped today is exactly what was captured. Linting it then found six problems
in code nothing had ever checked: object-truthiness null guards, a nullable
string conditional and a missing return type. All behaviour-preserving to fix,
but exactly the class of thing that ships silently when nothing is looking.

Also:

  - `config pi` gains --dry-run, the last agent without it.
  - The SessionStart hook installer now drops untagged duplicates of its own
    exact command — rows left behind before the marker existed, or by a suite
    that used to write into a real ~/.claude. Invisible in the UI; they just run
    the sync twice per session. A hook the user wrote is never touched, even one
    calling `mcpctl skills sync` with different flags.
  - docs/claude-integration.md and docs/prime-agent-extension.md, the two
    integrations that had no page.

prime-agent deliberately keeps its per-project MCP entry name rather than the
constant `mcpctl` claude and opencode now use: its switcher already unmounts the
previous project, so it never accumulates entries, and re-keying auth.json from
mcp:<project> to mcp:mcpctl would give up per-project token caching and needs a
migration. Documented as its own change rather than folded in here.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BVwuCjuMoA13gmzYEfcrNP

docs/prime-agent-extension.md

@@ -0,0 +1,101 @@
+# mcpctl × prime-agent
+
+## What `mcpctl config prime-agent --project X` wires up
+
+| Piece | Where | Purpose |
+|-------|-------|---------|
+| Proxy MCP | `~/.prime/agent/settings.json` | `{ type: "http", url: "<gateway>/projects/X/mcp" }` |
+| Bearer credential | `~/.prime/agent/auth.json` | `mcp:X` → a minted mcpctl PAT |
+| Skills | `~/.prime/agent/skills/` | the project's `SKILL.md` bundles |
+| `/mcpctl` switcher | `~/.prime/agent/extensions/mcpctl-switch.ts` | switch projects, and the active-project indicator |
+
+Unlike Claude Code (a stdio bridge, no token) and pi (native tools over
+JSON-RPC), prime-agent talks to the **HTTP gateway**, so the switch is really
+three things: a credential, a settings entry, and a reload.
+
+## Credentials are the fragile part
+
+`config prime-agent` mints an `mcptoken` per project, stores it under
+`mcp:<project>`, and retires the one it replaced. Three rules make that safe:
+
+- **A key being present proves nothing.** A revoked token would short-circuit
+  provisioning and leave prime-agent unable to reach the gateway while the
+  command reported success. mcptokens are shown once, so the stored token's
+  16-char `tokenPrefix` is compared against the project's *active* tokens rather
+  than sending the secret.
+- **No credential means the switch fails.** Non-zero exit, `settings.json`
+  untouched, so the previously active project keeps working instead of being
+  replaced by a mount that 401s — and the `/mcpctl` switcher, which reads that
+  exit code, reports failure rather than success over a project with no tools.
+- **Only the token we replaced is revoked.** Sweeping every `prime-agent` token
+  for a project would kill the one another machine is using. Anything else that
+  looks orphaned is reported, not deleted.
+
+This plumbing is shared with `config opencode`, parameterised by agent rather
+than copied.
+
+## The `/mcpctl` switcher
+
+`pi.registerCommand('mcpctl', …)` opens a picker, shells out to
+`mcpctl config prime-agent --project X --skip-extension --skip-marker`, then
+calls `ctx.reload()` — which re-reads `settings.json` and `auth.json` and
+rebuilds the MCP map, so the switch lands without restarting the app.
+
+`--skip-extension` stops it rewriting the very file it is running from;
+`--skip-marker` stops it re-scoping whatever repository prime-agent was started
+in, which Claude Code's own skills sync would then pick up.
+
+Above 20 projects it asks for a filter first: prime-agent's selector is an
+arrow-key list with no search, and real installs run to hundreds of projects.
+(opencode's dialog filters as you type, so its switcher needs no such prompt.)
+
+## The active-project indicator
+
+Published with `ctx.ui.setStatus('mcpctl', …)`, which prime-agent renders in its
+tray line next to the model name.
+
+Two quirks worth knowing:
+
+- prime-agent emits `session_start` **only from `reload()`**, never at startup —
+  so the indicator is also published on `turn_start`, the earliest moment with a
+  real UI context bound.
+- `resetExtensionUI()` clears extension statuses *after* `session_start`, so the
+  indicator set there is wiped before anyone sees it. It is re-published on a
+  short retry schedule to land after that reset.
+
+> The rendering itself only exists in prime-agent from
+> `prime-agent-extension-status.patch` (upstream PR pending). On an unpatched
+> build `setStatus` silently does nothing and no indicator appears.
+
+## The extension is real source now
+
+It used to exist **only** as a string literal inside
+`src/cli/src/config/prime-agent-extension.ts` — so nothing typechecked or linted
+it, which is precisely the gap that let a wrong `ctx.ui.select()` option shape
+ship in the pi extension.
+
+It now lives at `src/prime-agent-ext/mcpctl-switch.ts`, checked against the real
+`@earendil-works/pi-coding-agent` types:
+
+```bash
+pnpm typecheck:prime-agent-ext
+npx tsx scripts/generate-prime-agent-extension.ts   # after editing it
+```
+
+A test fails if the embed goes stale. Extracting it found six lint problems in
+code that had never been linted — all null-guard and return-type issues rather
+than live bugs, but exactly the class of thing that ships silently when nothing
+is looking.
+
+## What it deliberately does *not* do
+
+The MCP entry is still **named after the project**, not the constant `mcpctl`
+that `config claude` and `config opencode` now use. prime-agent's switcher
+already unmounts the previous project, so it never accumulates entries the way
+`config claude` did — the bug the constant name fixes does not exist here.
+
+The remaining difference is tool-prefix stability: switching changes tool names,
+so the model can hold stale ones. Moving prime-agent to a constant name would
+also re-key `auth.json` from `mcp:<project>` to `mcp:mcpctl`, giving up
+per-project token caching and needing a migration. Worth doing, but as its own
+change rather than folded into a parity pass.