feat(lifecycle): add implementWith for immutable execution profiles

[berenddeboer]

Operators who want a one-attempt backend/model experiment currently have to
change Repository or Harness defaults, and ordinary Work Items re-resolve
models on every Agent Turn. This adds the product API for Implement With
(#1033): a distinct implementWith command that freezes one complete
Explicit Work Item Execution Profile on the Work Item.

What changed:

• GraphQL implementWith takes repository, issue number, and a required
  complete profile (shipped Agent Backend, build model and optional Thinking
  Level, and either Same as build or an explicit review selection).
• The profile is persisted on work_item and projected on Work Item queries
  (backend, build/review, Thinking Levels, Same as build intent).
• Create revalidates Actionable Issue status, uniqueness, already-Active
  backend readiness, catalog membership, and Thinking Levels; failure
  creates nothing.
• Agent Turns use the frozen build or review selection. Later catalog drift
  fails closed before the Agent Backend CLI is spawned. Ordinary Implement
  Now / Locally / Queue / Intake / Implement All stay settings-resolved.
  Existing Work Items migrate with no profile.

Verification: lifecycle tests with real SQLite and stub backends
(implement-with, execution-profile decode/catalog); GraphQL
schema/resolver tests for the mutation, projection, and
INVALID_EXECUTION_PROFILE; migration and DbService suites. Review found
no findings. UI, activating a not-yet-Active backend, and settings-gate
narrowing remain sibling issues.

Closes #1033