refactor(kanban): extract models + schema into kanban_db_models (kanban_db.py god-file slice R1)

[andrexibiza]

Summary

kanban_db.py god-file slice R1: extract the models dataclasses + SCHEMA_SQL (window 904–1377, 474 lines) into hermes_cli/kanban_db_models.py. Part of the repo-wide large-file decomposition (tracker #78647).

What changed and why

• Task/Run/Comment/Attachment/Event dataclasses + SCHEMA_SQL moved byte-verbatim (golden blob 113e34842ec… verified; difflib delta 0)
• Stdlib-only (json/sqlite3/dataclasses); zero module-state deps; 9 external importers use the kb. namespace → 1-line re-export shim, zero caller edits
• Avoids refactor(kanban): extract board-path/integrity/policy mixins from kanban_db.py (shard s1) #79613's board-paths/integrity/policy territory (verified disjoint)
• Double-blind: 2 analysts → consensus → implementer → 2 blind re-reviewers (both APPROVED)

Testing

• 7 passed seam tests (identity ×6, no-star-import probe, from_row JSON parsing, schema 7 tables + executes)
• 38-file region: 19-failure set byte-identical to pristine base (pre-existing env class)
• ruff clean · git diff --check clean · LF-only · DCO signed

Coordination / interlock

• Epic: Part of [COMPLETE] Large-file decomposition: 20/20 done #78647 · Target: Part of Shard hermes_cli/kanban_db.py (god-file decomposition) #78632
• Sibling PRs: the 4 other kanban_db slices (txn, claims, spawnable, stats) — disjoint windows
• Dedup: no existing PR extracts the models

Part of #78647
Part of #78632