P6 during the run: split the runner task (02 core, 03 workers); guard.PreCheck as an owner commit; AGENTS.md: write code into files early

Task 02 (the whole runner) ended at 21:08 with no file and no commit: Ornith wrote the
implementation out in its reply (~700 lines), OpenCode compacted the session, and the model
then emitted a malformed tool call. It had also looked for the given test at a wrong path
("_files/…" relative to the plan: my ambiguity).

- 02-runner → 02-runner-core (types, Submit, Status, Touch, Cancel; no goroutines) and
  03-runner-workers; the given test split to match (runner_core_test.go gains three tests:
  queued storage, cancel while queued, Status never moves the lease); later tasks renumbered
  04-09 and every cross-reference checked.
- Copy paths spelled from the repo root.
- guard.PreCheck (+ its test) is committed here by the owner: the old task 05 told the
  implementer to edit internal/guard/, which AGENTS.md forbids. My contradiction.
- AGENTS.md rule 6: write code into files early, in small steps.
Both runner test files compile against a panic-only skeleton (the core file also alone) and
fail there on panic: todo.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/plans/p6/04-metrics.md

@@ -0,0 +1,83 @@
+# P6 task 04: `sift_batch_*` series in `internal/metrics`
+
+**Branch:** `p6` (`git switch p6`; `git status --short` must be empty, otherwise stop)
+**Commit subject:** `metrics: sift_batch_* series (jobs, targets, depth, in-flight, durations, bytes)`
+
+Add the batch series to `internal/metrics` so the caps (32 targets, 1 job/caller, 8 jobs total)
+and the byte budget can be re-sized from observed data. **This task touches only
+`internal/metrics`**: task 06 wires these methods to the runner's `batch.Hooks`.
+
+## Series (names and label values exact — the given test asserts these)
+
+| Metric | Type | Labels / meaning |
+|---|---|---|
+| `sift_batch_jobs_total` | counter | `outcome` ∈ `created`, `reused`, `done`, `partial`, `failed`, `cancelled`, `restart` |
+| `sift_batch_targets_total` | counter | `outcome` ∈ `ok`, `blocked`, `timeout`, `busy`, `upstream_4xx`, `upstream_5xx`, `unsupported`, `too_long`, `cancel` |
+| `sift_batch_queue_depth` | gauge | live jobs (`queued`+`running`) |
+| `sift_batch_targets_inflight` | gauge | targets currently being fetched |
+| `sift_batch_job_seconds` | histogram | wall time from submit to terminal status |
+| `sift_batch_target_seconds` | histogram | wall time per target |
+| `sift_batch_result_bytes` | histogram | stored bytes per target result |
+
+Classifying an error into a target outcome is task 06's job (it has the `*fetch.Error`); this
+package only records the label it is given.
+
+## Public surface (exact names — the given test compiles against them)
+
+```go
+// fields added to metrics.Set (registered in New)
+BatchJobsTotal       *prometheus.CounterVec // label "outcome"
+BatchTargetsTotal    *prometheus.CounterVec // label "outcome"
+BatchQueueDepth      prometheus.Gauge
+BatchTargetsInflight prometheus.Gauge
+BatchJobSeconds      prometheus.Histogram
+BatchTargetSeconds   prometheus.Histogram
+BatchResultBytes     prometheus.Histogram
+
+// methods (nil-guarded, like ObservePDF)
+func (s *Set) ObserveBatchJob(outcome string)
+func (s *Set) ObserveBatchTarget(outcome string)
+func (s *Set) SetBatchQueueDepth(n int)
+func (s *Set) SetBatchTargetsInflight(n int)
+func (s *Set) ObserveBatchJobSeconds(d time.Duration)
+func (s *Set) ObserveBatchTargetSeconds(d time.Duration)
+func (s *Set) ObserveBatchResultBytes(n int)
+```
+
+Both `outcome` counters fold any value outside their known set to `other` — the same "a raw string
+never becomes a label" guard as `ObservePDF`. Job outcomes: `created, reused, done, partial,
+failed, cancelled, restart`. Target outcomes: `ok, blocked, timeout, busy, upstream_4xx,
+upstream_5xx, unsupported, too_long, cancel`.
+
+## Rules
+
+1. A new file `internal/metrics/batch.go` adds the series to `metrics.Set` (registered in `New`),
+   with `Observe…`/`Set…` methods mirroring `ObservePDF`/`ObserveDigest`, nil-receiver safe.
+2. Both `outcome` counters fold any value outside their known set to `other`: a raw string
+   never becomes a label.
+3. The series are exposed through the existing `/metrics` and `/metrics.json` paths: no new
+   endpoint. `Snapshot`/`writeMetricsJSON` do not need to list them.
+4. Histogram buckets: seconds as the existing request histograms; bytes
+   `1K, 4K, 16K, 64K, 256K, 1M`.
+
+## Given
+
+Copy `docs/plans/p6/_files/metrics/batch_test.go` → `internal/metrics/batch_test.go`. It drives the `Observe…`
+methods, serves the `/metrics` handler (as `pdf_test.go` does), and asserts the exact series names,
+label values, gauge values, histogram counts, and that a raw outcome string is folded to `other`.
+
+## Steps
+
+- [ ] **1.** `git switch p6`; copy the given test. **2. See it fail.**
+- [ ] **3.** `internal/metrics/batch.go` and the registration in `New`.
+- [ ] **4.** `gofmt -w`; `go test -race -count=3 ./internal/metrics/` → `ok`. **5.** `make gate`.
+- [ ] **6.** Row `p6/04-metrics`; commit.
+
+```sh
+git add internal/metrics docs/implementer-log.md
+git commit -F .state/commit-msg.txt
+```
+
+## Done when
+
+- The given and existing metrics tests pass under `-race -count=3`; `make gate` ok; the given file is byte-identical.