feat(meta): expose effective experimental flag state and show it in Lab (#235)

## Related Issue

No issue. Third step of the subagent routing work. Stacked on #234
(merge #234 first; this diff includes its commit until then).

## Problem

`GET /meta` flattened experimental flags to booleans. The Lab tab showed
the saved `[experimental]` value, so when
`PYTHINKER_CODE_EXPERIMENTAL_*` (or the master switch) decided a flag,
the switch and the runtime disagreed with no explanation.

## What changed

- `FlagService.explain()` carries two server-decided fields:
`externallyControlled` (`source` is `env` or `master-env`) and
`overridden` (a saved value exists and differs from the effective
value). Nothing is inferred in the UI.
- `GET /meta` adds `experimental_flag_states[]` (`id`, `enabled`,
`source`, `config_value?`, `default_enabled`, `externally_controlled`,
`overridden`) beside the existing boolean map (additive).
- Web: the meta client maps the list (`toAppExperimentalFlagStates`),
workspace state stores it on every `/meta` refresh,
`usePythinkerWebClient` exposes `experimentalFlagStates` /
`experimentalFlagState(id)`, and the Lab rows show `Environment
controlled` and `Saved setting overridden` chips independently. The
switch still edits the saved setting.
- klient `flags` contract mirrors the two fields (contract parity test).
- `dist-web` rebuilt from source.

Tests: core `explain()` matrix (env+config agree → controlled only; env
overrides config → both; master-env; config → neither); gateway `/meta`
three cases; web mapper unit test; workspace-state stores the list;
Settings chips each independently and none for `source=config`.

## Checklist

- [x] I have read the
[CONTRIBUTING](https://github.com/PyModel/pythinker-code/blob/main/CONTRIBUTING.md)
document.
- [ ] I have linked a related issue (external PRs: the issue must have a
maintainer's `/approve`).
- [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**
* Lab settings now show when experimental options are controlled by the
environment or differ from saved settings.
* Experimental feature status is refreshed after configuration changes.
* **Bug Fixes**
* Prevented outdated server information from replacing newer settings
data.
  * Disabling the secondary-model pin now saves correctly.
* **Chores**
  * Updated the web application build and release metadata.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: elkaix

.changeset/lab-flag-effective-state.md

@@ -0,0 +1,5 @@
+---
+"@pymodel/pythinker-code": minor
+---
+
+The Lab settings show when an experimental flag is controlled by the environment and when the saved setting is overridden.

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

@@ -1,4 +1,4 @@
 {
-  "sourceHash": "de035986f5cd2b25cb86fe5a4fccb3b7cace86f47da191388a3b2485f2fc0093",
+  "sourceHash": "c3559f1b350b84cd73183ecca48c451946874033acb7af4738de205fdd4e4f7d",
   "sourceFileCount": 405
 }