… filter

Closes #126.

Introduces the `axis:<slug>` GitHub label namespace as pure plumbing so
operators can focus a sprint on a single topical concern (dispatch, cli,
observability, …) without changing how issues are queued.

Surface:
- `forge-loop status` now groups open `loop:ready` issues by `axis:*`
  label. JSON gains `axes` + `unaligned_count` + `axis_filter` keys.
  Issues with no axis land in an `unaligned` bucket and a yellow warning
  row is rendered in the human surface.
- `forge-loop run --axis <name>` (repeatable, union semantics) narrows
  dispatch to matching issues. Omitting the flag preserves pre-#126
  behaviour byte-for-byte.
- `forge-loop status --axis <name>` narrows the grouped view.
- The runner logs the active axis filter at startup and emits
  `axis_filter_active` / `axis_filter_empty` events per tick so a
  focused-sprint launch is auditable from `forge-loop events`.

Implementation:
- New `forge_loop.axis` module: pure helpers (extract_axes,
  group_by_axis, filter_issues_by_axes, parse_filter_env).
  Lowercase-normalises mixed-case labels; treats empty-slug labels
  (`axis:`) as unaligned.
- CLI threads `--axis` through env var `LOOP_AXIS_FILTER` so the tick
  loop can read it without a new kwarg on `run_loop`.
- Dispatch widens its `top_issues` fetch only when a filter is active,
  then trims back to `cfg.parallel` from the matched subset.

Docs: `docs/axis-labels.md` walks the namespace + edge-case matrix.

Tests:
- tests/test_cli_axis_filter.py — pure axis helpers, `status --json`
  payload shape, multi-axis dedup, Rich warning row, CLI flag wiring.
- tests/test_dispatch_axis_filter.py — single + union filters, regression
  guard for the no-filter path (identity-preserving), ready-gate priority,
  malformed-label safety, mixed-case normalisation, zero-match clean exit.