feat: add Gentle Shell as an independent Connect target

[rjoleinik]

Summary

Adds Gentle Shell as a first-class, independent Connect integration target (issue #7208). Gentle Shell reuses the existing Pi protocol and serialization with its own distinct paths, ownership rules, journal, registry entry, GUI surface, and refresh enrollment — it does not share or alias the Pi client.

• New src/clients/gentle-shell-path.ts with strict persistent-home validation (refuses shared/link/Pi/relative/malformed/unreadable destinations).
• Wiring across the integrations registry, catalog refresh, mutation plans, and ocx export --client (roster grows 19 → 20).
• CLI help and structure/ docs updated; GUI integrations tab, marks, overview, API, and FileIntegrationPage extended; i18n strings added for all 10 locales (en, de, es… zh, zh-TW, ja, ko, pt, ru, tr, vi).
• Tests for the gentle-shell client, path validation, integrations state, config export, GUI invariants, and layout fixtures.

Verification

Focused suites on this branch (based on dev @ b6dffa0), Bun 1.4.3:

• bun test tests/clients/gentle-shell-path.test.ts tests/clients/gentle-shell-client.test.ts tests/clients/integrations-state.test.ts tests/config/client-config-export.test.ts tests/test-layout.test.ts — 149 pass / 0 fail
• bun test tests/gui/integrations-invariants.test.ts — 51 pass / 0 fail (includes the "CLI names every client it supports" invariant)
• cd gui && bun test tests/integration-marks.test.ts tests/integrations-tab-coverage.test.ts — 13 pass / 0 fail

Screenshot of the new Gentle Shell integration page (asset per CONTRIBUTING.md pr-assets branch):

Checklist

• Scope stays focused and avoids unrelated cleanup.
• Docs or release notes were updated when needed.
• Security-sensitive changes were reviewed for secrets, auth, and unsafe defaults.

Refs #7208

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[lidge-jun]

Being carried into #7223 with your authorship preserved; this PR will be closed when the bundle lands.

[github-actions]

✅ Deterministic PR hygiene checks passed.

[github-actions]

⏳ DRAFT

• review readiness checklist open (0/4 boxes ticked).

What to do

• Tick all four boxes in the PR description once you're done (currently 0/4).

Review readiness checklist

• ⬜ Required local validation passed; commands, results, and any full-suite exception are documented.
• ⬜ I pushed my PR to a recent dev commit (at most 10 behind; a maintainer may still ask for the exact tip before merge).
• ⬜ I resolved all correct Codex and CodeRabbit findings.
• ⬜ My PR is ready for review.

0/4 boxes ticked.

This PR stays in draft until every box above is ticked.

[lidge-jun]

Landed on dev in a8d1bb8 via #7223 (your change is its own commit in that bundle, and the squash keeps your authorship as Co-authored-by). Thank you!