fix(serve): compute resume banner for created, reactivated, and forked sessions

[bug-ops]

Summary

• build_agent_factory now computes the resume-visibility banner (spec-068 section 13.5, AC-24) from the session's already-replayed message history, in its existing async prefix, and returns it alongside the build_agent closure — previously nothing on this path ever called SessionResumeInfo::from_messages.
• SessionActor::spawn stores the computed banner on a new SessionActorHandle::pending_resume_banner field; GET /sessions/:id/events renders it exactly once via the existing claim_resume_banner() guard, subscribing to the session's output broadcast before sending so a client that hasn't attached yet can never miss it. This gives claim_resume_banner() its first production call site.
• A forked session's banner never shows a "last active" timestamp: ForkEngine::fork stamps the new child session's row with the current time as part of its own bookkeeping, which would otherwise read back as a misleading "last active just now" on a session nobody has actually resumed yet. This matches the no-timestamp banner already shown for brand-new sessions created via POST /sessions.
• Covers all three entry points that funnel through build_agent_factory: session create, session reactivate, and session fork.

Closes #6425
Closes #6426

Test plan

• cargo +nightly fmt --check
• cargo clippy --profile ci --workspace --all-targets --features "desktop,ide,server,chat,pdf,scheduler,testing" -- -D warnings
• cargo nextest run --config-file .github/nextest.toml --workspace --features "desktop,ide,server,chat,pdf,scheduler" --lib --bins — 14276 passed, 0 failed, 35 skipped
• cargo test --doc --workspace --features "desktop,ide,server,chat,pdf,scheduler" — 19 passed, 0 failed
• Rustdoc gate (RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links") — clean
• New regression tests: exactly-once banner delivery across two concurrent/sequential attaches, end-to-end banner computation through build_agent_factory -> SessionActor -> events_session_handler, fork-specific banner (copied message count, no "last active" timestamp), show_banner = false negative path, persistence-disabled negative path, reactivate_session's own path
• .local/testing/playbooks/session-resume-history.md and .local/testing/coverage-status.md updated (main repo)