feat: stage Reader ingestion proposals durably

[jalehman]

What

Adds a durable, bounded staged-proposal protocol for Reader ingestion jobs, strengthens subagent context handling, and publishes review-before-write contracts for Granola, GitHub, and Gmail.

Why

Large filing plans and compacted retry context could produce ambiguous failures or unverifiable partial writes. Scoped ingestion needs a server-owned proposal that survives compaction, is tied to the exact artifact and capture page, and can be applied only after review.

Changes

• Stage proposal pages outside model receipts
• Bind proposals to job owner and artifact
• Enforce page, byte, and slug limits
• Freeze explicit per-job output budgets
• Preserve mutation evidence during compaction
• Publish staged Gmail ingestion behavior
• Clarify immutable Gmail capture revisions

Changes walkthrough

Area	Change
Proposal storage	Adds migration 136 and durable manifests
Agent submission	Preflights context and freezes execution limits
Tool loop	Compacts reads without source-like truncation claims
Ingestion skills	Separates propose and approved apply behavior
Tests	Covers limits, provenance, replay, compaction, and current fixtures

Testing

• bun test test/submit-agent.test.ts test/agent-job-proposals.test.ts test/gmail-thread-ingestion-skill.test.ts
• bun test on the ten repaired migration and contract fixtures
• bun run typecheck
• bun run verify
• Expected: 99 ingestion tests, 79 fixture tests, and 31 verification checks pass

The repository's sharded Test workflow is already red on master; the prior
PR head reproduced that same baseline plus one stale v134 migration-count
fixture, which this head repairs. Tier 1, Tier 2, JSONB parity, verify,
serial tests, gitleaks, semgrep, and both slow focused gates passed on the prior
head.