docs: spearhead-4half-and-5 specs + plans + kickoff prompts + AGENTS.md auto-spawn update

Phase A-B planning artifacts for the spearhead-4half-and-5 release:
- Strategic plan (.omo/plans/): ADR-0010 deviation, 3-dev dispatch shape,
  auto-spawn flow via task(subagent_type=general, run_in_background=true)
- Slice 4½ spec + plan: rutster-sim crate, SimAudioPipe, LatencyProbe,
  ConcurrencyRunner, TickLagGauge, sim-bench CI gate
- Slice 5 spec + plan: G711Codec, TwilioMediaStreamsServer, TrunkSession,
  trunk_driver::drive, MediaLeg enum, CallControlClient trait
- 4 kickoff prompts (PM + dev-a/b/c) updated for auto-spawn framing
- AGENTS.md PM launch checklist item 4: auto-spawn-dev-via-task() flow

Signed-off-by: Aaron D. Lee <himself@adlee.work>

AGENTS.md

@@ -275,6 +275,8 @@ and "why isn't X here?" questions. Consult it before adding anything.
 
 ## Multi-agent coordination — the relay (cross-model)
 
+> **Default for routine task splitting:** prefer subagent dispatch via `task(subagent_type=..., load_skills=[...], run_in_background=...)` from the main Sisyphus session — it natively isolates worker context and saves tokens without any message-bus setup. This section applies only to the long-haul multi-dev *persistent-session* paradigm where each dev needs its own visible streaming terminal.
+
 When more than one agent session works this repo in parallel (the PM / senior-dev "lift"
 paradigm), they coordinate through a small MCP message-bus — the **relay** at
 [`~/Sources/relay`](file:///home/alee/Sources/relay). One server, many terminals: each
@@ -480,16 +482,28 @@ This live-tails both `inbox.log` (dev → pm messages) and `poller.log` (poll cy
 markers + nudges), so you see dev activity in real time without depending on the PM
 agent surfacing it. You see what the PM sees.
 
-**4. Bring up the PM/dev sessions (manual / tmux / kitty):**
+**4. Launch the PM session (single terminal — the PM auto-spawns the devs):**
 
 ```bash
-~/Sources/relay/start.sh --repo ~/Sources/rutster --port 7110 --kitty   # or --tmux
+~/Sources/relay/start.sh --repo ~/Sources/rutster --port 7110 --tmux
+# Opens a tmux session "relay-lift" with two windows: relay (the server log)
+# + pm (a fresh claude session). Attach: tmux attach -t relay-lift
 ```
 
-`--kitty` / `--tmux` spawns 5 terminals (relay + pm + dev-a/b/c) with the MCP relay
-pre-registered for each. The generated prompts bake in `RELAY_PORT=7110` so the shims
-(`call.py`) self-configure even without the env. (For `--manual`, the relaying prompts
-are printed for you to paste into separate terminals.)
+The user pastes ONLY the PM kickoff prompt (from
+`docs/superpowers/kickoffs/<release>-pm-prompt.md`) into the `pm` window. **The PM is
+responsible for spawning the 3 dev subagents** — it dispatches each as a long-running
+background `task(subagent_type="general", run_in_background=true, prompt="<dev-prompt-file-
+contents>")`. The dev subagents inherit the kickoff prompt as their initial context; they
+read the plan + spec + their worktree, dispatch `task(subagent_type="kimi-worker", ...)` per
+plan task, and stream STATUS reports back to the PM via the relay.
+
+(Previous flow opened 5 tmux windows + required the user to manually paste 4 prompts —
+one per dev session. As of 2026-07-05 the harness supports subagent spawning for users,
+which collapses the manual-paste step into a single PM auto-dispatch. If the older manual
+flow is preferred, `start.sh --kitty` / `--tmux` still opens the 5-window layout and the
+dev prompts at `docs/superpowers/kickoffs/<release>-dev-{a,b,c}-prompt.md` can be pasted
+as before.)
 
 ### PM-mode discipline (load-bearing — this was a real failure mode)