fix(worktree): unify session workspace lifecycle

[beruro]

Summary

• model session workspace launch as three explicit modes: local, create isolated worktree from an optional base ref, or reuse a registered linked worktree
• keep the TypeScript/Zod/Rust/CLI contract in parity and return authoritative worktree path, checkout branch, and base ref
• scope UI selection by repository, reject stale async results, and preserve existing-worktree paths from the picker through launch
• validate reused worktrees before persistence and distinguish borrowed checkouts from session-owned worktrees during merge, discard, rollback, and deletion
• bound setup commands and diff/source caches; isolate GitHub source data by endpoint, connection, credential source, username, and repository
• include architecture and frontend UI audit reports; no new visual styling or layout was introduced

Verification

• cargo check -p org2
• cargo test -p git worktree --lib — 34 passed
• cargo test -p agent_core workspace_launch --lib — 4 passed
• targeted Vitest suite — 4 files, 65 tests passed
• ESLint on all changed TypeScript/TSX files
• git diff --check

Known verification limits

• full TypeScript typecheck still reports the pre-existing ContextInfoButton.tsx:468 string | undefined error; that file is unchanged from develop
• the new git_api cache-cap unit test is present, but the crate's macOS lib-test target is blocked by an existing unconditional import of a Windows-only helper; production cargo check passes
• no rendered Tauri E2E was run

UI audit

• 2 pre-existing fix candidates, deferred to a dedicated UI cleanup
• 19 keep-with-reason findings
• 3 systematic abstraction candidates