Support safe existing repository adoption

[suyog19]

Summary

• prevent initialization and upgrades from overwriting repository-owned guidance
• add an explicit local_context model and reviewed mature-repository adoption flow
• keep generated assistant bootstraps compact while linking canonical policy and contextual local documents
• validate ownership boundaries, missing references, stale generated files, and safe upgrade behavior

Why

Existing repositories may already contain important workflow, architecture, UX, deployment, and validation guidance in AGENTS.md or CLAUDE.md. Initialization previously replaced those files unconditionally, making adoption destructive.

Impact

Mature repositories can now preserve existing guidance verbatim with --adopt-existing-context, declare it in the manifest, validate it, and carry it safely through upgrades. Existing override mechanisms remain authoritative for policy-level configuration and cannot weaken locked controls.

Validation

• 45 pytest cases passed
• 19 acceptance checks passed
• production-adoption dry run against suyog19/suyog19.github.io preserved both assistant files byte-for-byte and validated with the frontend profile

Closes #1