fix: close context-budget integration seams

[Calmingstorm]

$## Summary\n\nCloses all six blockers from the PR #277 campaign integration review:\n\n1. freezes agent breaker admission to the captured provider/model identity\n2. shares one generation plan and budget snapshot across agent soft compaction, request, and rescue\n3. makes autonomous-loop stop cancellation-settled before reporting stopped\n4. distinguishes saved restart-bound context ceilings from boot-effective runtime targets in API and WebUI\n5. binds ceiling migration provenance to canonical config identity across aliases and sibling configs\n6. persists canonicalized schema-owned override maps atomically through generic Config Admin\n\nEach integration reproduction is pinned. ui/dist is rebuilt.\n\n## Validation\n\n- full suite under coverage: 9,561 passed, 6 skipped\n- npm run check: passed\n- coverage gate: 0 findings, 89.9% reported\n- lint gate: 0 findings\n- type gate: 0 findings\n- apply-registry gate: 0 findings\n- git diff --check: clean\n\nNo deployment or campaign merge performed.

[Calmingstorm]

LGTM — approved at bba2e7d. All six integration fixes verified with biting mutations against your reproduction pins, clean controls on each: (M1) reverting the breaker to the live provider fails the frozen-provider pin; (M2) ignoring the generation-plan provider fails the shared-snapshot seam pin; (M3) dropping task.cancel() fails the stop-waits-for-settlement pin while the self-stop pin correctly still passes; (M4) resolving effective targets from the saved ceiling fails three runtime-truth pins; (M5) constant config identity fails both the alias-rendezvous and distinct-config pins; (M6) re-walking raw map keys fails both canonical-persistence pins including the restart round-trip. Design checks: the loop-stop contract (cooperative event + task cancellation + await-settlement, honest self-stop wording), the saved-vs-boot ceiling split through the shared _boot_codex_group_status machinery, and the validated-map one-leaf persistence rule all read correctly. Independent gates at bba2e7d: suite 9,562 passed / 5 skipped, npm run check exit 0 with byte-current dist, coverage/lint/type/apply-registry clean, diff-check clean, CI green. Merging to the campaign branch.