feat(directives): unified contracts surface — backbone (#128)

This is the backbone PR for the unified directive workflow. A directive
holds a sequence of contracts; each contract is a spec body whose
execution drives tasks in the directive's shared worktree. Lifecycle
(Lock & Start, queue scheduler, drag-reorder) lands in follow-ups.

What's in this PR:
- Migration adds `position` (queue order) and `merge_mode`
  (shared|own_pr) columns to directive_documents. The actual table
  rename is deferred — the legacy `contracts` table from the old
  contracts system still exists, and the rename collision waits for
  Phase 5 to drop legacy contracts.
- Repository: list orders by position; create assigns next-position;
  update accepts merge_mode; new reorder_directive_document_position
  shifts siblings inside a transaction.
- HTTP: endpoints aliased under /api/v1/directives/{id}/contracts and
  /api/v1/contracts/{id}/... with a new /contracts/{id}/reorder.
- Frontend: api types renamed `DirectiveContract*` (avoiding the
  legacy `Contract` type collision); document-directives.tsx imports
  via aliases so the rest of the file is untouched.

Internal struct + table names stay `DirectiveDocument` /
`directive_documents` until the legacy contracts cleanup.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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