feat(collector): add session snapshot v1 contract

[luhe19001]

Summary

• publish the session-snapshot/v1 canonical wire contract, schema, and fixtures
• map private sessions through an explicit allow-list with bounded fields, stable hashes, and privacy-safe redaction metadata
• deterministically fit optional evidence under the accepted 256 KiB aggregate cap
• add real session-start extraction, allow-list census coverage, and content-free corpus measurement tooling

Size and privacy evidence

• 506 approved local sessions: p50 6,266; p95 29,519; p99 82,361; max 210,488 bytes
• zero degradation and zero rejection under the 262,144-byte cap
• excludes raw transcripts, raw commands, tool output, diffs, synthesis state, absolute paths, and credentials

Verification

• go test ./...
• go vet ./...
• deterministic valid/invalid/boundary fixtures and exact limit/+1 tests
• aggregate reduction stages covered independently

Compatibility

• breaking wire changes require v2
• the bounded agent identifier remains open for additional agents
• server adoption is intentionally handled by a separate dependent PR

[luhe19001]

Superseded by a smaller, merge-ordered review stack:

1. #83 — canonical session start
2. #84 — public v1 contract
3. #85 — private-to-wire export boundary
4. #86 — aggregate payload fitting
5. #87 — fixtures, measurement, and docs

Each PR is independently scoped, has a reviewer guide, and is green in CI. Merge them in order.

No work was discarded: the original monolithic head is preserved on backup/eng-1340-session-snapshot-monolith, and this PR's source branch remains available for comparison or rollback.