feat: reveal saved plans in file manager (#208)

## Related Issue

N/A — internal product work.

## Problem

Saved plan files are outside the active workspace, so their paths cannot
be opened in the workspace preview.

## What changed

- Added a server-owned reveal action that resolves the saved plan by
session, agent, and tool call identifiers and confines it to that
session plan directory.
- Added **Reveal in file manager** actions to saved plan UI surfaces.
- Documented the saved plan location and the REST endpoint.

## Checklist

- [x] I have read the CONTRIBUTING document.
- [x] I have linked a related issue, or explained the problem above.
(N/A — internal product work.)
- [x] I have added tests that prove my feature works.
- [x] Ran `gen-changesets` skill, or this PR needs no changeset.
- [x] Ran `gen-docs` skill, or this PR needs no doc update.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
  * Added **Reveal in file manager** for saved plan files.
* Supports plans stored outside the workspace and works for active or
previously saved sessions.
  * Added safeguards to ensure only valid plan files are revealed.
* **Documentation**
  * Documented saved plan locations and the plan-reveal API.
* **Bug Fixes**
* Improved plan display by separating the file path from the
file-manager action.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: elkaix

.changeset/reveal-saved-plans.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": minor
+---
+
+Add an option to reveal saved plan files in your file manager. Select Reveal in file manager from a saved plan.

apps/pythinker-code/dist-web/.web-bundle-manifest.json

@@ -1,4 +1,4 @@
 {
-  "sourceHash": "b937f8282ae21a3bac9a26b32ea3182d28e2ac95f5d4c9484678ff9569338f82",
+  "sourceHash": "965c37d0a139928c9e6107941d28eb228a6be4d212d1bbd110338a83ef8646a2",
   "sourceFileCount": 404
 }