feat(dsh): isolate multi-tab sessions and device leases

[HarveySang]

What changed?

• Scope root tasks and client snapshots by DSH session, with exact task/attempt identities and stale-callback guards.
• Separate per-session device preferences from all-or-nothing multi-device leases. Reject conflicting ownership and make stale stop/release requests harmless.
• Bind nested agents to their original task, cancel on owning-session/plugin disposal, and keep tab switching independent of task lifetime.
• Keep the managed browser globally serial with an exact owner and session-scoped install approval/status.
• Update local HTTP contracts, regression tests, reload compatibility, and English/Chinese/Japanese behavior documentation.

Why?

Allow independent DSH sessions to use non-conflicting phones without sharing messages, task state, errors, stop actions, or device ownership. Preserve single-session multi-phone execution.

Testing

Re-run before this commit:

• cd deepseek-harness-plugin && rtk pnpm run check: 45 test files / 290 tests passed; type checks, builds, and plugin structure validation passed.
• rtk npm pack --dry-run: passed.
• rtk git diff --check and staged diff check: passed.

Automated checks do not establish real dual-device concurrency.

Device verification

Pending — keep this PR in draft and do not merge or release until the real-device gate passes.

• Two authorized Android phones on supported DSH: A/B run concurrently; rapid switching and new C do not cross messages, state, or errors.
• Stop A without affecting B; delayed old stop and old-attempt release cannot affect A's replacement task.
• Background errors remain in the owning session; same-phone contention reports busy; one session can still execute across all selected phones.
• A owns the browser: B's browser task reports busy while an independent phone task can run.
• Confirm the DSH Host actually permits concurrent sessions; a globally serial Host fails this acceptance gate.

Notes

One atomic implementation commit based on 674e3589. Target main advanced to 6ff5b04 during preparation; this branch was not rebased or merged with it. CI and mergeability are tracked separately.

No dependency, configuration, credential, migration, version, tag, or release changes. Preferences and leases are process-local. The existing running DSH instance was not replaced. Rollback is a revert of this PR.