Finish ACP session resume and persistence docs

[NathanFlurry]

What changed

• Adds ACP resume orchestration for persisted sessions, including native session/load / session/resume and transcript fallback via a one-shot preamble.
• Persists outbound user prompts and captured session updates so actor SQLite can reconstruct session transcripts after sleep/wake.
• Adds session resume regression coverage, OpenCode fallback/native resume coverage, public export coverage, and related protocol/client updates.
• Adds website architecture docs under Reference > Advanced > Architecture > Sessions & Persistence and records architecture-doc maintenance guidance in CLAUDE.md.
• Removes accidentally committed local secure-exec link: / Cargo path dependency state, returning the workspace to just secure-exec-status pinned mode.
• Tightens ACP post-spawn cleanup so create/resume failures kill the adapter process before returning, and reports terminal disposal as abnormal exit instead of clean exit 0.

Why

RivetKit actors can survive sleep/wake, but the live VM and ACP agent process do not. This PR makes persisted Agent OS sessions resumable by rebuilding durable context from actor SQLite and rehydrating a live ACP session on demand.

Validation

• git diff --check
• pnpm --dir packages/core build
• pnpm --dir website build
• just secure-exec-status reports mode: pinned
• Local managed-dependency scan shows no external @secure-exec/* or @agent-os-pkgs/* link: deps. The remaining pnpm-lock.yaml link: entries are local workspace packages in this repo.

Blocked locally:

• cargo test -p agent-os-sidecar acp -- --test-threads=1 cannot resolve secure-exec-sidecar-browser from crates.io.
• pnpm --dir packages/core exec vitest run tests/session-resume.test.ts tests/opencode-session.test.ts --reporter=verbose reaches the same Cargo resolver failure while building agent-os-sidecar.

The unresolved crate is not local-dev state: secure-exec-sidecar-browser is still listed in the workspace dependency manager and CI, but is not published on crates.io. That needs a secure-exec publish or a deliberate Agent OS browser-wrapper dependency change before Cargo validation can pass in fully pinned mode.

[railway-app]

🚅 Environment agent-os-pr-1493 in rivet-frontend has no services deployed.