#18966: fix(config): downgrade unknown bundled plugin references to warnings

## Problem When a bundled plugin's directory is missing or unresolvable (e.g. after upgrading from an external plugin to a stock/bundled one), config validation treats the plugin reference as a hard error and blocks startup. This affects users who have `plugins.entries`, `plugins.allow`, or `channels` referencing a plugin like `feishu` that was previously external but is now bundled — if the `extensions/` directory is incomplete, the application refuses to start. ## Solution Downgrade "plugin not found" and "unknown channel id" errors to warnings when the referenced ID is clearly associated with a configured channel plugin: - **Channel validation**: when an unknown channel key is also referenced in `plugins.entries` or `plugins.allow`, emit a warning instead of a hard error - **Plugin entries/allow/deny validation**: when an unknown plugin ID matches a channel key in `config.channels` or a channel declared by a discovered plugin, emit a warning instead of a hard error The warning message includes guidance that the entry can be removed. ## Test Plan - Added test case that simulates a missing bundled plugins directory (`OPENCLAW_BUNDLED_PLUGINS_DIR` pointed to an empty dir) - Verifies `feishu` referenced in `plugins.entries`, `plugins.allow`, and `channels` produces warnings (not errors) and `res.ok === true` - All 362 existing config tests pass with zero regressions <!-- greptile_comment --> <h3>Greptile Summary</h3> Downgrades "plugin not found" and "unknown channel id" config validation errors to warnings when the referenced ID is cross-referenced in both channel and plugin configuration. This prevents startup failures when a bundled plugin's directory is temporarily missing (e.g., after migrating from an external to a stock/bundled plugin). - `src/config/validation.ts`: Channel validation now emits a warning (instead of an error) when an unknown channel key is also referenced in `plugins.entries` or `plugins.allow`. Plugin `entries`/`allow`/`deny` validation similarly downgrades to a warning when the unknown plugin ID matches a configured channel key or a channel declared by a discovered plugin. A new `isKnownChannelPlugin` helper centralizes this check. - `src/config/config.plugin-validation.test.ts`: Adds a test simulating a missing bundled plugins directory (`OPENCLAW_BUNDLED_PLUGINS_DIR` pointed to an empty dir), verifying that `feishu` referenced across `plugins.entries`, `plugins.allow`, and `channels` produces warnings (not errors) and `res.ok === true`. <h3>Confidence Score: 4/5</h3> - This PR is safe to merge with minimal risk — it only relaxes validation from hard errors to warnings for a specific, well-scoped scenario. - The changes are well-structured and narrowly scoped. The validation downgrade only applies when there is a clear cross-reference between channel config and plugin config, making accidental bypasses unlikely. Existing tests pass and a new test covers the target scenario. One minor test hygiene suggestion was noted (clearing the registry cache in the finally block) but it does not affect correctness. - No files require special attention. _{Last reviewed commit: c2b384a} <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->