fix: remove gr2 identity binding (boundary enforcement)

[laynepenney]

Summary

• Removes gr2 identity binding code from public repo (recall#637)
• Identity resolution (workspace.toml, agent.toml, g2_ prefix) is premium per boundary policy
• Preserves SYNAPT_AGENT_ID env var as the neutral OSS seam
• Code will be migrated to synapt-private in a future sprint

Removed:

• Gr2Context/Gr1Context/StandaloneContext dataclasses and detection functions
• gr2 branches in _agent_id() and _resolve_griptree()
• workspace column from presence schema, migration, and all queries
• test_gr2_identity.py (416 lines)
• TestWorktreeInWho from test_channel_worktree.py

Test plan

• Full test suite: 1961 passed, 0 failed, 19 skipped
• Team review: Apollo, Atlas, Sentinel confirmed no dependencies

Premium boundary: recall is OSS (channel infrastructure). Identity resolution is premium.

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[laynepenney]

Obsolete after the recall history rewrite. The public OSS identity-binding path was removed from main and the remaining capability was ported into premium/private code instead of being restored in OSS.