c-accel: arch prereqs — compat contract, C-side strategy, build scaffolding

Three companion specs under docs/specs/ formalize the boundaries
established in docs/c-accel.md, unblocking bd-mq7 (descriptor → C
plan compiler):

* c_accel_compat.md (bd-47e) — pinpoints what must stay byte-equal
  between PB_ENABLE_C unset and =1: public surface, generated
  module wrappers, 64-bit cdata, WKT shapes, unknown fields,
  extensions, errors. Calls out the lazy-view exclusion.

* c_accel_strategy.md (bd-z7x) — pb_plan struct layout, field-name
  luaL_ref caching, 4 KB stack-backed pb_buf, cached per-field
  stack indices (the 2× win from spike Phase B), sub-buffer over
  backpatching, map/oneof/unknown handling.

* c_accel_build_packaging.md (bd-wky) — where the C module lives
  (runtime/pb/c/), how it builds, what the rockspec gains, the CI
  matrix shape.

Scaffolding that lands now:

* runtime/pb/init.lua — PB_ENABLE_C=1 opt-in pcall hook; the
  loaded module (or nil) is exposed as pb.c_runtime for
  introspection. Silent fallback when the module is absent.

* Justfile — `build-c` / `clean-c` recipes (stub erroring cleanly
  until bd-ra6 lands runtime/pb/c/), new lua_cpath constant,
  LUA_CPATH wired through `test` and `test-one`.

* .builds/{pure-lua,c-enabled}.yml — sourcehut CI manifests, one
  per activation mode (sourcehut has no matrix; parallel jobs go
  in separate files). ubuntu/noble images.

* .sourcehut/conformance.yml — outside .builds/ so it doesn't
  auto-submit; trigger manually with `hut builds submit` before
  releases.

* .gitignore — runtime/pb/c_runtime.{so,dylib} and runtime/pb/c/*.o.

745/745 tests pass with PB_ENABLE_C unset and PB_ENABLE_C=1
(silent fallback verified).

Closes bd-47e, bd-z7x, bd-wky. Unblocks bd-mq7.

tooling: add Beads workspace + beads agent skill

Checks in the portable parts of the Beads issue tracker that
this project now uses as its durable source of truth (per the
PLAN.md retirement). The Dolt DB internals (.beads/embeddeddolt/,
.beads/backup/, runtime sockets/locks/sync-state) stay
gitignored by .beads/.gitignore; what lands here are:

  .beads/.gitignore       — Beads-managed ignore rules
  .beads/README.md        — workspace README from bd init
  .beads/config.yaml      — workspace config (id, etc.)
  .beads/metadata.json    — workspace metadata
  .beads/issues.jsonl     — passive JSONL export of issues
  .beads/interactions.jsonl — passive log of state changes

The JSONL files are the human-readable / diffable surface; the
authoritative store remains the Dolt DB synced via refs/dolt/data
on the git remote (see Beads SYNC_CONCEPTS.md).

Also adds .agents/skills/beads/ — the beads agent skill that
tells subagents to use `bd` for task tracking instead of local
TODO files. Mirrors the rule already in CLAUDE.md but in a form
other agents and IDEs can pick up.

Closes the implementation side of bd-2nl (which removed the
.git/info/exclude rule that previously hid .beads/ from Git).