Add existing project adoption workflow

[itseffi]

Summary:

• Add productize init existing with deterministic project scanning, dry-run/force/json support, and project markdown generation.
• Promote accepted ADRs and shared workflow memory into .productize/project decisions.
• Wire project knowledge into PRD, TechSpec, task creation, task execution prompts, skills, README, and docs.
• Restore the release workflow contract required by the repository verification tests.

Verification:

• make verify passed: fmt, lint (0 issues), 3658 tests / 3 skipped, build.