[codex] Improve local plugin development workflow (#5821)

## Thinking Path

> - Paperclip is the control plane for autonomous AI-agent companies.
> - Plugins are the extension point for adding capabilities without
expanding the core product surface.
> - Local plugin development needed a tighter CLI-first loop so plugin
authors can scaffold, run, install, inspect, and reload plugins without
reaching into internal package paths.
> - The server plugin install path also needed local-path handling that
keeps plugin identity, dashboard routes, and development watchers
coherent.
> - This pull request adds the CLI scaffold/install workflow, fixes the
server and SDK edge cases that blocked that loop, and updates the
agent-facing plugin creation skill and docs.
> - The benefit is that contributors can develop plugins from local
folders with a documented, repeatable happy path.

## What Changed

- Added `paperclipai plugin init` coverage and CLI wiring for local
plugin scaffolding.
- Improved local plugin install handling, plugin key route resolution,
dashboard capability behavior, and dev watcher startup/reload behavior.
- Fixed plugin SDK worker entrypoint validation for symlinked package
layouts.
- Added targeted tests for plugin init, server plugin authz/watcher
behavior, SDK worker host validation, and the authoring smoke example.
- Added a short local plugin development guide and refreshed the plugin
authoring guide plus `paperclip-create-plugin` skill instructions.

## Verification

- `pnpm run preflight:workspace-links && pnpm --filter
@paperclipai/plugin-sdk build && pnpm --filter
@paperclipai/create-paperclip-plugin typecheck && pnpm --filter
paperclipai typecheck && pnpm --filter @paperclipai/plugin-sdk typecheck
&& pnpm --filter @paperclipai/server typecheck`
- `pnpm exec vitest run --project paperclipai
cli/src/__tests__/plugin-init.test.ts`
- `pnpm exec vitest run --project @paperclipai/plugin-sdk
packages/plugins/sdk/tests/worker-rpc-host.test.ts`
- `pnpm exec vitest run --project @paperclipai/server
server/src/__tests__/plugin-dev-watcher.test.ts --pool=forks
--poolOptions.forks.isolate=true`
- `pnpm exec vitest run --project @paperclipai/server
server/src/__tests__/plugin-routes-authz.test.ts --pool=forks
--poolOptions.forks.isolate=true`
- `pnpm --dir packages/plugins/examples/plugin-authoring-smoke-example
test`
- Confirmed `pnpm-lock.yaml` is not included in the PR diff.

## Risks

- Medium risk: this touches plugin install routing, CLI command
behavior, and the local development watcher.
- Local path plugin installs execute trusted local code by design; the
new docs call out that trust boundary.
- No database migrations are included.

> For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and
discuss it in `#dev` before opening the PR. Feature PRs that overlap
with planned core work may need to be redirected — check the roadmap
first. See `CONTRIBUTING.md`.

## Model Used

- OpenAI Codex, GPT-5 coding agent, tool-enabled local shell and git
workflow, medium reasoning effort. Context window details were not
exposed in this runtime.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] If this change affects the UI, I have included before/after
screenshots
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

UI screenshots: not applicable; this PR changes CLI/server/plugin docs
and tests, not board UI rendering.

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

server/src/services/plugin-dev-watcher.ts

@@ -164,6 +164,10 @@ export function createPluginDevWatcher(
   const watchers = new Map<string, FSWatcher>();
   const debounceTimers = new Map<string, ReturnType<typeof setTimeout>>();
   const fileExists = fsDeps?.existsSync ?? existsSync;
+  log.info(
+    { resolvesInstalledPlugins: Boolean(resolvePluginPackagePath) },
+    "plugin-dev-watcher: initialized",
+  );
 
   function watchPlugin(pluginId: string, packagePath: string): void {
     // Don't double-watch
@@ -293,11 +297,23 @@ export function createPluginDevWatcher(
   }
 
   async function watchLocalPluginById(pluginId: string): Promise<void> {
-    if (!resolvePluginPackagePath) return;
+    if (!resolvePluginPackagePath) {
+      log.debug(
+        { pluginId },
+        "plugin-dev-watcher: no package path resolver configured, skipping lifecycle event",
+      );
+      return;
+    }
 
     try {
       const packagePath = await resolvePluginPackagePath(pluginId);
-      if (!packagePath) return;
+      if (!packagePath) {
+        log.debug(
+          { pluginId },
+          "plugin-dev-watcher: plugin is not a local-path install, skipping watch",
+        );
+        return;
+      }
       watchPlugin(pluginId, packagePath);
     } catch (err) {
       log.warn(