#1201 Keep background commands' startup image alive across daemon exits

Fixes the Linux merge-queue failure where a background command never started: sandbox_unavailable: ... openat2 secure walk failed for .../run/aft-pinned-<pid>-<start>: No such file or directory.

Cause. In daemon mode AFT pins its startup executable into <storage>/run/aft-pinned-<pid>-<start>/ and launches sandbox-launch and the gh shim from it (#1151). When the daemon exits on connection loss, its clean-exit path removes that directory. A detached background task whose sandbox-launch was still applying its profile then pointed at a folder that no longer existed. Crash-restart cleanup of dead pins had the same lifetime mismatch, and any background command outliving a restart lost its gh shim.

Fix.

• Each native task hard-links (or byte-copies) the startup image and gh shim into its own task bundle before either sandbox backend builds its profile, so the profile and the child environment name paths the task owns. A daemon exit or restart can remove its own pin without breaking detached tasks.
• A live daemon's pin holds a kernel owner lease, so dead-pin cleanup can't delete it on misleading PID or start-time data.
• A missing daemon pin is rebuilt from the held startup-image handle and logged, never from a replacement binary at the deploy path.
• Explicitly configured binary targets are unchanged.

Tests. scope_end_kill::connection_loss_preserves_pin_until_delayed_sandbox_and_shim_run holds the sandbox start across a daemon exit and failed before the fix. Unit tests cover cleanup against a live process, the lease overriding a mismatched clock, task images outliving the daemon pin, and repair from the held image. Landlock probes ran on the build server's new image, and the Seatbelt delayed-launch window passed on a macOS build machine. A separate test-only commit retries ETXTBSY when the replacement test executes a freshly copied binary (a sibling test's fork can briefly hold the copy open for writing).

Build server (main merged): 215 pin/sandbox/child-env lib tests, 15 scope_end_kill tests, 3 executable-replacement tests three times, formatting and the Windows check passed. No agent-visible text, tool schema or config change.

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Summary by cubic

Fixes detached background tasks losing their startup executable when the daemon exits or restarts mid-sandbox-start.

• Each native task now hard-links the startup image and gh shim into its own task bundle before the sandbox profile is built.
• Daemon pin cleanup now honors a kernel owner lease, so dead-pin cleanup can't remove a live daemon's pin on misleading PID or start-time data.
• A missing daemon pin is rebuilt from the held startup image handle, never from a replacement binary at the deploy path.
• Adds a test-only start gate that holds sandbox-launch across a daemon exit, reproducing the original failure; includes an ETXTBSY retry for the replacement-executable test on Linux.

Migration

None. Explicitly configured binary targets and non-daemon hosts are unchanged.