Add session-fenced collaborative claims

[Deicyde]

Moved from #34. Its head branch lived on VivienCabannes/autoform-bot, which is being deleted, so this PR uses the same-named branch in facebookresearch/autoform-bot (same commits, head e511514). Earlier review and discussion are on #34.

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Summary

• add explicit claim acquire, renew, release, list, and cleanup commands
• fence collaborative leases with exact Git-ref CAS and per-worktree session receipts
• separate durable article claims from resource claims and migrate legacy refs conservatively
• pin local repositories and shared scratch state to retained directory generations
• bound Git transfer, object promotion, subprocess time and output, scratch storage, ref batches, messages, and retries
• reject implicit stealing and avoid remote mutation outside explicit claim commands

Depends on #28. The reviewable change is commit e511514; earlier commits belong to the dependency stack.

Validation

• focused claims and CLI suite: 241 passed
• Python 3.13 full suite: 829 passed, 4 skipped
• Python 3.10 full suite: 830 passed, 3 skipped
• installed-wheel local bare and file:// acquire/list/release smoke tests
• make lint
• make check-example
• git diff --check
• independent exact-diff review: SHIP

[Deicyde]

Closing as stale rather than attempting a 35-conflict wholesale rebuild. The source-snapshot dependency landed and was hardened through #95; basic claims, atomic multi-target operations, and current transport/scratch behavior landed through main, #135, and #126.

The unique session-fencing work is still valuable, but exact head e511514 predates the arbitrary multi-target claim_target/helper model, lacks batch semantics, and retains the six previously confirmed correctness blockers. Local repaired descendant 729ae642 is useful as a regression corpus, not a merge candidate. A fresh follow-up should port those tests and redesign exact-generation fencing plus bounded transport against the current claim contract.