feat(scout): the 24/7 scout is an exit-3 stub in the Go binary while its LaunchAgent still runs it

[linhdmn]

Problem

The automatic queue writer is dead in the shipped binary. devagent scout (FR-SCOUT-01, "24/7 scout: research backlog -> PRD -> queue") returns the exit-3 notPortedError for every mode except --replay (internal/cli/actions_scout.go:25-27), while scripts/install-scout-launchagent.sh still installs a LaunchAgent whose ProgramArguments run devagent scout --repo … --interval … — so the 24/7 scout has been failing on every tick, and scout.heartbeat.json goes stale with nothing to explain it.

The deterministic core was ported long ago (BuildScoutPrompt, ExtractScoutPayload, ParseScoutOutput, FallbackTask, heartbeat, single-instance lock in internal/scout/scout.go); only the live cycle is missing — and its two stated blockers (the queue package, the worker runtime) landed since: internal/queue (FR-GO-04 #194) and internal/workers (FR-GO-05).

Consequence for the factory: internal/tracker/tracker.go:1-5 documents the intended triangle "scout writes the queue, builder consumes it", but with the scout leg dead the queue only ever receives rows a human enqueues through POST /dispatch, so the builder lane starves into LLM self-selection (see #355).

Requested change

Port runScoutOnce / runScoutLoop into internal/scout and wire the existing flags (--once, --interval, --worker, --timeout, --repo, --dry-run):

1. lock → config (scout.{worker,model,maxQueued,intervalMinutes}) → queue-depth check before dispatch (dispatch is the paid step) → prompt → workers.GetWorker(name).Spawn → ExtractScoutPayload → ParseScoutOutput → queue.EnqueueTask (task-PRD sidecar included) → heartbeat → release lock.
2. An empty ResultText with a nonzero exit code is an error, never a silent empty payload (the stale-binary class, loops 285-290); unparseable output falls back to the deterministic per-UTC-day FallbackTask id; ErrAlreadyQueued is a dedupe hit, not a failure.
3. --dry-run runs the documented no-AI-call preview: every step except the enqueue/heartbeat writes, and no worker spawn.
4. Remove the stale "live dispatch is NOT ported" claims from internal/scout/extract.go and scout.go package headers.

Acceptance

• devagent scout --once enqueues exactly one row from a canned payload, writes .devagent/prds/<id>.md + scout.heartbeat.json, and a second identical cycle reports deduped with no new row.
• A live foreign scout.lock holder ⇒ locked, zero writes.
• Queue at scout.maxQueued ⇒ queue-full and no dispatch.
• devagent scout --once --dry-run creates no files; --replay output stays byte-identical.

[linhdmn]

Implemented in the working tree 2026-09-14 (labels adjusted so the loop does not re-pick it before this lands).

• internal/prdintake + internal/loopdriver/intake.go + devagent prd-intake + internal/commands/updown.go (up/down) + internal/scout/run.go (live FR-SCOUT-01 cycle).
• Verified: gofmt/go vet/golangci-lint clean, full go test ./... green, and live smoke runs — an open - [ ] item in a scratch repo's docs/PRD.md was ingested, claimed by the next iteration, and published as .selfbuild/goals/loop-1.md; up proved the detached driver (holds the loop lock, iteration 2, phase queue-empty) and failed it with the cause (exited: max iterations reached) when the driver halted; down stopped the recorded pid and left no process behind; devagent scout --once --dry-run previews a cycle with no AI call and no writes.