Isolate Slack thread sessions with worktrees

[Innei]

Summary

• add a GitThreadWorkspaceManager that creates stable per-thread worktrees from the repository default branch
• insert worktree resolution between workspace resolution and session persistence so new Slack threads get isolated cwd while resumed threads reuse their saved worktree
• add worktree cleanup with configurable retention/interval, protecting any worktree still referenced by sessions and pruning expired unbound managed worktrees
• update agent git workflow prompt rules and stale session workspace fallback tests

Verification

• pnpm -F @innei/kagura exec vitest run tests/thread-worktree.test.ts tests/conversation-pipeline.test.ts tests/workspace-resolution.test.ts tests/prompt-memory-context.test.ts
• pnpm -F @innei/kagura typecheck
• pnpm -F @innei/kagura build

[safedep]

SafeDep Report Summary

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