fix(instance): release resources at worktree boundaries

[wqymi]

Problem

Isolated worktrees can finish or be removed while their directory-scoped Instance remains cached. The stale Instance keeps subprocesses and other scoped resources alive, including after the directory itself has disappeared.

Fix

Add bounded, directory-addressable Instance teardown and invoke it when an isolated agent settles and before worktree removal. The captured cache entry is removed before cleanup, replacement instances are protected from late teardown, missing paths are still reclaimed, and session CWD overrides are cleared only for the owning directory.

Verification

• 50 focused tests passed; 1 platform-only test skipped
• Package and push-hook TypeScript checks passed
• Lint completed with 0 errors (3 pre-existing workflow warnings)
• Added coverage for blocked disposers, replacement-instance races, missing worktree paths, retained worktrees, and cross-directory session CWD isolation

Post-Deploy Monitoring & Validation

• Watch logs for instance dispose did not complete and unexpected increases in long-lived child processes grouped by removed worktree directories.
• Healthy signal: completed/removed worktrees no longer retain directory-scoped subprocesses, while resumed worktrees initialize once and unrelated session CWDs remain unchanged.
• Failure signal: teardown warnings repeat for the same directory, workflow completion gains sustained ~2s latency, or child-process counts continue growing after worktree removal.
• Validate over the first 24 hours; owner: MiMo Code maintainers.
• Roll back this commit if teardown warnings become frequent or worktree reuse/session path behavior regresses.

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