feat(gmail): add gmail extract-attachments command

[newhoggy]

Summary

Closes #1510.

• Adds omni-dev gmail extract-attachments --output-dir PATH [--dry-run] [--quiet] [-o FORMAT], retroactively extracting attachments for messages gmail sync/sync-all already archived, without re-fetching from Gmail.
• gmail sync --extract-attachments only ever extracted attachments for messages fetched during that run — presence-on-disk is the archive's idempotence mechanism (ADR-0064), so turning the flag on never backfilled already-archived mail, even under --full. ADR-0065 named this gap explicitly; until now the only fix was deleting the affected .eml files and re-running --full --extract-attachments.
• Purely local and offline: reads the manifest and .eml files already under --output-dir, never resolves a GmailClient — dispatched alongside sync-all's no-client special case in GmailCommand::execute, before credential resolution. --account is rejected the same way sync-all rejects it.
• Reuses gmail sync --extract-attachments's extraction path verbatim against on-disk bytes: trusts attachment_count > 0 as a fast-path filter (the same cheap heuristic scan sync always runs), skips any record whose attachments/ directory already exists (idempotent, safe to re-run), and calls the exact same MIME-parse-and-write helpers fetch_and_write_one uses (manifest_path/write_atomic promoted to pub(crate) — no new writing logic needed).
• Per-message failures accumulate into a report and never abort the batch (ExtractAttachmentsReport, mirroring SyncReport's compute-render-decide shape from ADR-0064 Decision 4); a real-parser/heuristic disagreement is a silent no-op, not an error.
• --dry-run parses every candidate for an accurate count rather than echoing the heuristic, and writes nothing.
• See ADR-0065's amended Consequences section and docs/gmail.md#extract-attachments for the operator-facing docs.

Commit-by-commit:

1. feat(gmail): add gmail extract-attachments command — the engine, report types, CLI wiring, and reused sync/manifest helpers.
2. docs(docs,release): document gmail extract-attachments, amend ADR-0065 — ADR amendment, docs, changelog.

Test plan

• cargo build
• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo test (full suite) — all pass except the two pre-existing, unrelated daemon_test.rs lifecycle-flake tests (zero files touched by this branch overlap with daemon)
• cargo insta test --test integration_test — help_all_output snapshot reviewed and accepted for the new subcommand
• New unit/CLI tests covering candidate filtering, idempotence, --dry-run accuracy, per-message error accumulation, heuristic/parser disagreement, and CLI dispatch routing (no client resolved, --account rejected)

[github-actions]

Coverage

Total: 97.31% 🔴 0 pp vs main

Comparing 4a657b9..3a67d71 (merge-base → PR head)

File	Before	After	Δ
src/cli/gmail/extract_attachments.rs	—	95.02%	🆕 new
src/cli/gmail/extract_attachments/engine.rs	—	98.01%	🆕 new
src/cli/gmail/extract_attachments/report.rs	—	92.31%	🆕 new
src/cli/gmail.rs	98.33%	98.23%	🔴 -0.1 pp
src/cli/gmail/sync/engine.rs	97.58%	97.65%	🟢 0.07 pp
src/cli/gmail/sync/manifest.rs	97.96%	98.24%	🟢 0.28 pp

Patch coverage

Patch: 96.9% (562/580 new lines covered)

File	Patch	Uncovered new lines
src/cli/gmail.rs	97.67% (42/43)	151
src/cli/gmail/extract_attachments.rs	95.02% (191/201)	111-113, 129-132, 134-135, 138
src/cli/gmail/extract_attachments/engine.rs	98.01% (197/201)	92-95
src/cli/gmail/extract_attachments/report.rs	92.31% (36/39)	28-30
src/cli/gmail/sync/engine.rs	100% (65/65)	—
src/cli/gmail/sync/manifest.rs	100% (31/31)	—

Uncovered new lines (18)

• src/cli/gmail.rs:151
• src/cli/gmail/extract_attachments.rs:111
• src/cli/gmail/extract_attachments.rs:112
• src/cli/gmail/extract_attachments.rs:113
• src/cli/gmail/extract_attachments.rs:129
• src/cli/gmail/extract_attachments.rs:130
• src/cli/gmail/extract_attachments.rs:131
• src/cli/gmail/extract_attachments.rs:132
• src/cli/gmail/extract_attachments.rs:134
• src/cli/gmail/extract_attachments.rs:135
• src/cli/gmail/extract_attachments.rs:138
• src/cli/gmail/extract_attachments/engine.rs:92
• src/cli/gmail/extract_attachments/engine.rs:93
• src/cli/gmail/extract_attachments/engine.rs:94
• src/cli/gmail/extract_attachments/engine.rs:95
• src/cli/gmail/extract_attachments/report.rs:28
• src/cli/gmail/extract_attachments/report.rs:29
• src/cli/gmail/extract_attachments/report.rs:30

_{📦 Full per-file coverage summary · run summary}