feat: add Herdr workspace launching

[corwinm]

Summary

• add explicit --herdr support to arashi switch and arashi create
• add configured and exact HERDR_ENV=1 launcher selection
• resolve Git-backed non-bare source identity for each selected worktree
• invoke herdr worktree open with an argument array and validate its JSON response
• preserve existing launcher precedence, JSON restrictions, and created-worktree failure semantics
• update generated schema/contracts and local CLI documentation

Validation

• pnpm test — 96 files, 1006 passed, 1 skipped
• pnpm run lint — 0 errors
• pnpm exec tsc --noEmit
• pnpm run build
• pnpm run schema:check
• pnpm run contract:check
• real Herdr v0.7.4 open/reuse smoke test
• independent Codex review completed; two source-identity findings fixed and re-reviewed

Related pull requests

• CLI: feat: add Herdr workspace launching #98
• Docs: docs: document Herdr workspace workflows arashi-docs#50
• Skills: docs: add Herdr guidance to Arashi skill arashi-skills#41
• Meta/OpenSpec: feat: add Herdr workspace launch integration arashi-arashi#224

Part of corwinm/arashi-arashi#222.

[chatgpt-codex-connector]

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