exec: v1.4 notify, simplified: no background contact after release

[jaredLunde]

Revises the v1.4 "Sandbox job notify" spec (module doc of exec_endpoint), FLEET.md's "Job-end callbacks", the v1.4 fixtures and the coverage test together. Nothing is implemented yet; this is the contract the implementation tasks are held to.

The design rule now: after a job is released, the replica contacts the sandbox only on real use: a callback, the session's own use of its sandbox, a bash_job call, or the next open. Nothing runs on a timer. The one accepted difference from today is stated up front: an end no callback delivers (a dead or stopped sandbox, every attempt failed) is noticed at the next real use instead of within seconds.

Removed

• The 5-minute sweep: any periodic contact keeps the sandbox awake.
• The per-keep generation in the URL: a release now needs a successful keep plus a callback inside the window, which already proves the same URL and token.
• The notify key set (upcoming/retired keys, the rotation procedure): the replica holds one seal secret, and a token under an old one gets 503 and falls back to next use.
• The exited-only-with-leftovers and open-reconcile-spares-tracked-jobs requirements, which guarded things today's code doesn't guarantee either.
• The proof's own reconcile: a release now starts a live reconcile, so a proof answers immediately.

Changed

• Every end a reconcile finds is read by today's follow, so the notice, the failure policy and the error notice are today's. A sandbox that cannot be listed sends its released jobs back to today's follow.
• A listed exited is applied as a stream's shell_exit is, so today's per-job record decides whether leftovers are reported. That includes today's one case that does report them: a background job that a bash_job call put back while its shell still ran.
• The job ledger is entries in the transcript's fenced segment batch, written in the same batch as the end's message. reset_for_new_session drops the entries.
• A callback is answered once the list is handed over, or 503 at 20 s. At most 16 callbacks wait per session.
• Callbacks must carry exactly one Content-Length and no Transfer-Encoding. A duplicated session_id/tenant is refused at both the edge and the replica. Callbacks are handled only at /_beyond/agent, and every refusal has a fixed body.
• The baseline notifier is one pinned curl invocation (wget is dropped).

The spec section goes from 576 to about 505 lines, with 50 requirements instead of 64. exec_notify_vectors now models the proof, a proof that overtakes keep, released jobs listed as exited, ended, lost or missing, an unreachable sandbox, duplicate query parameters and request framing. The previous test fails against the new fixtures (4 of 10), and the new one passes 10/10.

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