chore: release 0.12.0 — DID onboarding tools + approval_code two-step flows

[H179922]

Sync from the private tree after audit-remediation Phase A shipped (@multimail/mcp-server@0.12.0 is already live on the registry).

• New tool request_did_challenge + optional agent_did/did_challenge_id/did_signature on create_account — bind an Ed25519 did:key agent identity at signup
• update_account / delete_account two-step approval flows — optional approval_code parameter (202 pending_approval → resubmit with code)
• server.json + package.json + changelog → 0.12.0

🤖 Generated with Claude Code

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[H179922]

Codex gate flagged stdio↔worker mirror drift (Sharp 1). Resolution: the worker mirror lives in the private repo and shipped in the same change set — private PR #180 (merged 7d0a590) updated mcp-worker/src/index.ts with request_did_challenge (13 refs) and both approval_code flows (34 refs), deployed to prod via the green deploy-production run, live worker serving 0.12.0. This public repo mirrors the stdio server only, so the mirror rule is satisfied upstream — finding inapplicable to this diff. Merging with override.