feat(instances): add multi-repo agent sessions

[binikingi]

Summary

• Multi-repo mode: a single session can now operate across multiple repositories simultaneously, giving the agent access to all repos without context loss
• UI: "m" hotkey toggles multi-repo mode; stacked path list with inline editing, Tab autocomplete, and parent-dir pre-fill when adding paths
• Worktree layout: all worktrees are created inside a single persistent parent dir at ~/.agent-deck/multi-repo-worktrees/<branch>-<id>/, named after each repo (e.g., server/, webapp/). This co-locates all repos under the agent's cwd, eliminating cross-repo permission prompts
• Without worktree: multi-repo sessions use the same parent dir layout with symlinks to the original repos
• Docker sandbox: mounts each repo under /workspace/<dirname> with automatic name deduplication
• Persistence: extends existing tool_data JSON blob — no schema migration needed
• Fork support: multi-repo state (paths, worktrees, parent dir) propagates correctly to forked sessions
• Cleanup: deleting a session removes git worktrees and the persistent parent dir

Directory layout example

# Multi-repo + worktree (branch: implement-changes)
~/.agent-deck/multi-repo-worktrees/implement-changes-eeea2993/
├── server/      ← git worktree (branch: implement-changes)
└── webapp/      ← git worktree (branch: implement-changes)

# Multi-repo without worktree
~/.agent-deck/multi-repo-worktrees/a1b2c3d4/
├── server/      ← symlink → ~/projects/server
└── webapp/      ← symlink → ~/projects/webapp

Test plan

• go build ./cmd/... ./internal/ui/... ./internal/session/... ./internal/docker/... ./internal/statedb/... — compiles without errors
• go test ./internal/ui/... — all tests pass
• go test ./internal/docker/... — all tests pass (including new WithMultiRepoPaths tests)
• go test ./internal/statedb/... — all tests pass (including new marshal/unmarshal round-trip tests)
• go test ./internal/session/... -run "MultiRepo|Deduplicate" — all new unit tests pass
• go fmt — no formatting issues
• Manual: toggle multi-repo in new dialog, add 2+ paths with Tab autocomplete, submit session
• Manual: verify worktrees created under ~/.agent-deck/multi-repo-worktrees/
• Manual: verify agent cwd is the parent dir with repo subdirs
• Manual: delete multi-repo session, verify parent dir and worktrees cleaned up
• Manual: fork a multi-repo session, verify state propagated

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