feat: ubuntu-22.04 runner coverage (phase 2 slice D)

[phramz]

Summary

Final Phase-2 slice. Moves the builder runner from `ubuntu-24.04` (noble) to `ubuntu-22.04` (jammy) so compiled bundles run on both OS releases via glibc forward-compat. Forces a full rebuild of all 182 bundles by folding `BUILDER_OS` into `ComputeSpecHash`.

What lands

• `builders/common/builder-os.env` pins `BUILDER_OS=ubuntu-22.04`.
• `internal/planner.ComputeSpecHash` gains `builderOS` parameter; every spec_hash now depends on it.
• `build-php-core.yml` + `build-extension.yml` dispatch to `ubuntu-22.04` / `ubuntu-22.04-arm`.
• `runtime_deps.linux` rewritten to jammy-native package names (drop `-t64` suffixes).
• `compat-harness.yml` gains `runner_os` fixture dimension + 5 `bare-jammy-` fixtures (x86_64 only).
• README: supported runners + imagick-on-noble known gap.

Known gap — imagick on noble

imagick compiles against `libMagickWand-6.Q16.so.6` (jammy's SOVERSION). Noble ships SOVERSION 7 via `libmagickwand-6.q16-7t64` — no transitional back-port. Users on `ubuntu-24.04` runners who request `extensions: imagick` hit a clear apt-install failure. Tracked in a follow-up issue (to be filed post-merge).

Test plan

• CI: planner emits all 182 cells (every spec_hash changed from `BUILDER_OS` addition).
• CI: 182 jammy rebuilds succeed; wall-clock dominated by 10 × grpc ≈ 45–60 min.
• CI: lockfile auto-commits 182 refreshed digests.
• CI: 70 harness fixtures (65 noble + 5 jammy) pass with no new allowlist beyond existing bands.
• CI: `make check` passes.

Spec: `docs/superpowers/specs/2026-04-22-phase2-ubuntu-22.04-design.md`

[phramz]

Closing without merge. Slice D's approach (a') — build on jammy, rely on glibc forward-compat to run on noble — doesn't hold because PHP core --enable-intl links against a specific libicu SOVERSION, and Ubuntu noble ships libicu74 only (no libicu70 back-port). Jammy-built PHP binaries can't load on noble. Same class of problem as the imagick SOVERSION gap flagged in the spec. Rolling back; the real cross-OS story will be designed separately — see follow-up issue.