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.
• Fix slow-poll starvation and stale browser ownership. Reconcile durable, session-owned command outcomes so immediate device-busy failures do not become misleading launch timeouts or disappear from the sidebar.
• 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.

Accepted verification gate

The maintainer explicitly approved multi-phone automated logic verification plus single-phone real-device acceptance for this merge. Two simultaneous physical phones are not required for this PR's revised gate.

双设备并行已验证逻辑，未做双真机实测。 Automated concurrency checks do not establish real dual-device concurrency or prove that every supported Host permits simultaneous real-device tasks.

Automated verification

Verified candidate source at 2c1dcf2d8af8d2693bcf664a468ad54bc370720e in an isolated worktree:

• pnpm run check: 45 test files / 301 tests passed, including type checks, builds, and plugin structure validation.
• npm pack --dry-run and actual tarball creation: passed.
• git diff --check and staged diff check: passed.
• Coverage includes concurrent sessions, same-session root rejection, all-or-nothing multi-device acquisition, conflicts without partial leases, exact stop, stale stop/attempt release, nested-agent isolation, cancellation/disposal, client generations and background state/error isolation, HTTP ownership/same-origin checks, and global browser-owner serialization.
• The immediate command-failure regression was reproduced on the real test Host; four new tests failed before the fix and passed afterwards.
• Latest push and pull-request CI: Node 22.19.0 / 24 checks and packaged-plugin compatibility across all four supported DSH versions passed.

Single-phone real-device acceptance

Used one authorized Android phone with DSH 0.1.1-rc.2, in a separate test profile loaded from the candidate tarball. Installed client-bundle hashes matched the tested build. No model/phone execution was mocked.

• Execute a phone task: inspect the system Settings home page, report its title, and return to the launcher; completed successfully.
• Switch sessions and Chat/OpenGUI views during execution; the owning task remains active, its device selection stays locked, and its stop controls remain scoped.
• Create a new session without cancelling the background phone task or copying its messages, errors, or stop control.
• Compete for the same phone from another session: show an immediate device-busy error only in that session; the owner keeps running. Switching back or creating another session does not display that error.
• Stop through the real UI, then immediately start a replacement task in the same session.
• Replay the stopped task's exact HTTP stop request: receive 409; the replacement task and attempt remain active and unchanged.
• The replacement phone task completes, the Host returns an empty task batch, the stop button disappears, and the phone is back at the launcher.

Real dual-phone concurrency, real multi-phone batch execution, and real browser-versus-phone competition remain unverified physically; their ownership and concurrency logic is covered by automated tests. They are not claimed as real-device acceptance here.

Delivery boundaries

• One atomic PR; rollback is a revert of this PR, with no data migration or recovery step.
• No dependency, configuration, credential, version, tag, or release changes. Preferences and leases are process-local.
• No WorkBuddy files changed or included.
• The existing DSH instance was not replaced or restarted; only the isolated test Host loaded the candidate.
• This merges source only. A package release is a separate, unrequested action.