fix(proposals): colliding propose_page no longer silently overwrites durable pages

[kai392]

Summary

• Root cause: approve() exempted every PAGE proposal from the artifact collision guard and routed any existing page id through update_page (vault-edit path from bug: vault_to_kb proposes page edits with wrong id — slugifies title instead of preserving existing page id #219). A same-title slug collision or malicious slug_hint silently wiped durable body and scope on approval, including under the starter trusted-agent auto-drain.
• Fix: Page updates are opt-in via propose_page(..., update_existing=True). vault_to_kb sets the flag. Bare colliding propose_page raises the same "already exists" error claims always did. Not exposed on MCP/JSONL.
• Impact: Accidental or malicious page collisions can no longer destroy approved knowledge. Vault edits keep working.

Test plan

• Collision refuse / update_existing opt-in / deleted-target refuse
• Vault-edit + delete-suite green locally
• CI green

Risk / tradeoffs

• Callers that relied on undocumented "id exists → update" for non-vault edits will now get a collision error (correct). Only vault_to_kb is the intentional update path.

[kai392]

@coderabbitai review