docs(security): define local credential vault architecture

[SarthakWade]

Summary

• define the unsigned local/community credential tier
• require operating-system user presence for every saved credential use
• keep credential management local-user-only and agent access alias-based
• define origin, process, threat, persistence, and future signing boundaries
• record the delivery order for Authentication profiles: durable login state across restarts #155, Credential vault: secure storage and origin-bound account aliases #156, and Authentication orchestration: challenges, account selection, login, and save consent #157

Security decisions

• no silent credential use in unsigned builds
• no plaintext vault fallback
• no password values in argv, MCP, responses, logs, flows, diagnostics, or temporary files
• no normal-profile credential access from private contexts
• Developer ID and notarization remain future release work in E7 — Cut a release — everything since v1.0.2 is unreleased #45

Verification

• pnpm test
• pnpm test:runtime
• pnpm test:npm
• pnpm --filter @headless/web lint
• pnpm build
• git diff --check

Closes #154

[SarthakWade]

Superseded by the concurrently opened #171, which records the approved owner decision and reserves the correct ADR number around #169 and #170. Continuing #155 as a stack on #171.