feat(ui): load right panel plugin manifests

[pascalandr]

Summary

• Add a typed right-panel plugin manifest loader.
• Let bundled plugin manifests contribute right-panel tabs and Status sections through the existing registry.
• Run onLoad/onUnload lifecycle hooks when the RightPanel mounts/unmounts.
• Keep the manifest list explicit and empty by default; no arbitrary external code loading in this PR.

Stacking

• Base branch: feat/right-panel-customization.
• Stacked on feat(ui): add right panel customization #615. After feat(ui): add right panel customization #615 merges, retarget this PR to dev.

Validation

• npm exec --no -- tsx --test packages/ui/src/components/instance/shell/right-panel/plugin-manifest.test.ts packages/ui/src/components/instance/shell/right-panel/registry.test.ts
• npm run typecheck --workspace @codenomad/ui
• git diff --check
• npm run build --workspace @codenomad/ui
• final gatekeeper pass: no findings

[pascalandr]

Stack note: this is intentionally based on feat/right-panel-customization so the diff only contains the manifest/lifecycle loader. The repository restrict-non-dev-prs workflow currently blocks non-dev base branches for authors outside ALLOWED_NON_DEV_PR_ACTORS, so CI is skipped until #615 merges and this PR is retargeted to dev, or the allowlist includes this author.

[pascalandr]

Rebased on the updated #615 branch after the right-panel customization popover UX change. Manifest loader diff remains stacked and clean.

Validation after rebase:

• npm exec --no -- tsx --test packages/ui/src/components/instance/shell/right-panel/plugin-manifest.test.ts packages/ui/src/components/instance/shell/right-panel/registry.test.ts
• npm run typecheck --workspace @codenomad/ui
• git diff --check
• npm run build --workspace @codenomad/ui
• final gatekeeper pass: no findings

[pascalandr]

Update: rebased this stacked branch on the latest feat/right-panel-customization and force-with-lease pushed a7eee0e.
Validation passed after rebase:

• npm exec --no -- tsx --test packages/ui/src/components/instance/shell/right-panel/plugin-manifest.test.ts
• npm exec --no -- tsx --test packages/ui/src/components/instance/shell/right-panel/registry.test.ts
• npm run typecheck --workspace @codenomad/ui
• npm run build --workspace @codenomad/ui
• git diff --check
  Expected CI note: restrict-non-dev-prs still blocks this stacked PR while its base is not dev.