ah-nyl.4 runner: pi+zellij implementation + ahub-run supervisor Past Stand

Implement internal/runner + cmd/ahub-run per docs/SPEC.md SS9 (read SPEC fully; SS6, SS9 normative). This is the trickiest package — the SS9 contract (worktree layout, .task/ files, status precedence) is normative; follow it to the letter.

Deliverables:
- internal/runner: New(cfg *config.Config, logger *slog.Logger) *PiZellij implementing ports.Runner (compile-time check var _ ports.Runner = ...).
  - Start: create/reuse worktree + branch per SS9 (git -C <repo> worktree add ...; handle existing worktree dir and existing branch for retries), write .task/PROMPT.md and .task/meta.json, worktree-local ignore via the resolved git-dir info/exclude per SS9, ensure zellij session (zellij attach --create-background task-<id>), spawn the pane (zellij --session task-<id> run --cwd <worktree> -- ahub-run --task-id N --attempt K --report-url <url> -- pi --mode json -p @.task/PROMPT.md --model <model> --no-skills --no-extensions [pi_args...]). Return RunInfo.
  - Status: precedence per SS9 — exit.json => Completed (outcome success/error by code); else session alive in `zellij list-sessions --short` => Running with LastEvent=mtime(events.jsonl); else Completed with OutcomeCrashed, exit -1. Parse PiSession + CostUSD from events.jsonl leniently: scan lines as loose JSON maps; session id from the first object that has a plausible session identifier; cost accumulated from usage/cost fields when present. IMPORTANT: pi 0.70.2 is installed locally — empirically capture a real `pi --mode json -p 'say hi'` JSONL sample (any cheap/configured model, or ask for the shape via `pi --help` + a dry attempt; if no model is invocable offline, mark the parser 'best-effort, fixture-based' and derive fixtures from pi's documented event shape), commit the sample as a testdata fixture, and pin the parser to it.
  - Kill: zellij kill-session + best-effort delete-session. Summary: git log/diff per SS9.
  - ALL zellij/pi/git argv construction centralized in commands.go with unit tests asserting exact argv (SS9 requirement).
- cmd/ahub-run: supervisor per SS9 contract — tee child stdout to .task/events.jsonl (stderr passthrough), atomic exit.json (tmp+rename), best-effort POST to --report-url (2s timeout, 1 retry), signal-death -> 128+sig, --keep-pane default true iff $ZELLIJ set (then print resume hint + exec $SHELL), false => exit with child code.

Tests: stub `zellij`/`pi`/(where sensible `git` is real — use real git with a t.TempDir() repo for worktree tests) as executable scripts prepended to PATH; cover: worktree create+reuse, argv construction, status precedence matrix (exit.json / alive / gone), events parsing from fixture, ahub-run end-to-end via os/exec (tee, atomic exit.json, report POST to httptest, exit-code mapping). No sleeps >100ms; no real zellij sessions in tests.

Constraints: work ONLY under internal/runner/ and cmd/ahub-run/. No go.mod changes, no other packages, no .beads/. Build/test ONLY yours: go build ./internal/runner/... ./cmd/ahub-run/... && go vet <same> && go test <same>. Commit 'runner: ...' staging only your paths. Do NOT push.