feat(activity): E6 · origin kill-switch + home waiting strip (#186)

[hellno]

Summary

Finishes the two remaining Activity/home gaps in the v4 request-origin epic and wires the stop_brake widget E1 left dead-coded for exactly this. The Activity STOP is reframed to the shared three-state kill-switch driven by whether an agent is live (disabled grey No agents running → amber Stop all agents → red Confirm STOP…), the Activity header becomes the quiet uppercase ACTIVITY label with no hero title/subtitle, and the wallet home gains a one-line "Waiting on you" strip (N waiting for you · Review → amber when pending, else Nothing waiting for you.). One shared Shell::has_active_agent() predicate now backs both the kill-switch and the home agent-presence acting/idle label so they can never contradict, and brake_state() is a pure, unit-tested mapping. STOP stays reachable via ⌘K from every surface even when the header shows the disabled idle marker, and no approval/zeroize path is touched.

Linked issue

Closes #186

Definition of Done

• cargo fmt --all --check is clean
• just check is green — clippy -D warnings on both the default config and --features tray
• cargo test --workspace is green
• No new or changed dependencies in Cargo.toml / Cargo.lock
• Visual/UI change follows DESIGN.md (amber = human, cyan = agent; matches the deckard-v4.html golden ref for .acthead / .killswitch / .waitstrip)
• Secrets stay in Zeroizing and are never logged or Debug-printed (no secret-handling code touched; STOP zeroize logic unchanged)

Evidence (paste command output here)

$ cargo fmt --all --check
# clean (exit 0, no output)

$ just check
cargo clippy --locked -p deckard-app --all-targets -- -D warnings
    Finished `dev` profile [unoptimized + debuginfo] target(s)
cargo clippy --locked -p deckard-app --all-targets --features tray -- -D warnings
    Finished `dev` profile [unoptimized + debuginfo] target(s)
cargo clippy --locked -p deckard-signerd --all-targets --features dev-signerd-bin -- -D warnings
    Finished `dev` profile [unoptimized + debuginfo] target(s)
# exit 0, no warnings

$ TMPDIR=/tmp cargo test --workspace
# 418 passed, 2 ignored across 38 suites (exit 0)
# incl. new widgets::tests::brake_state_arm_beats_agent_state ... ok

CI: all 5 checks green (linux, macos, quick, cargo-deny-advisories, cargo-deny-supply-chain).

Notes for reviewers

• Trust: STOP is never gated to the header — ⌘K revoke-all (aliases stop/panic/kill/halt/emergency/freeze) fires stop_revoke_all unconditionally from every surface, and the agent surface keeps its own always-live STOP. The idle header marker is disabled presentation only.
• Adversarial review: codex (GPT-5, xhigh) cross-model pass returned ACHIEVED (0 P0/P1) — verified the daemon makes revoked && pending unreachable (lock() denies all pending + sets revoked atomically), so the kill-switch can't falsely read "No agents running" mid-flight. Its one actionable P2 (kill-switch vs agent-presence predicate mismatch) is fixed here via the shared has_active_agent().
• Deliberate follow-up (not a blocker): the home strip count — like the existing sidebar needs-you badge it mirrors — only refreshes while on the Activity surface (seeded at unlock). Making the pending count globally live is a separate change that also touches the shipped sidebar badge.
• ⚠️ Screenshots PENDING: E6 is a GUI change and the views sit behind the unlock gate, which this agent shell can't drive (documented macOS input limitation). Capture from a foreground just demo (or just qa, passphrase deckard-qa): Home strip empty + pending; Activity header STOP in Idle / Ready (just demo-agent) / Armed.