ci: port the latest Claude actions from documentation

[Troublor]

The bug

The 2026-07-30 migration (#5) said the actions were copied verbatim from
megaeth-labs/documentation. They were — but from that repo's 2026-07-20
state, not its main. The copied claude-pr-review/action.yml is byte-identical
to documentation 0de71e1 (2026-07-20), ten days and ~10 commits behind
dc68b9c (2026-07-28).

The other three actions were unaffected — claude-interactive,
claude-issue-triage, and claude-label-check are byte-identical either way.
claude-pr-review was not.

Live symptom. mega-agents and mega-e2e were repointed here on 2026-07-30 and
both pass github_identity_token, an input the stale copy never declared. Actions
drops undeclared inputs with a warning, so the review falls back to the job token:
no unified identity, and no review-thread resolution. From mega-agents run
30596025629,
today:

##[warning]Unexpected input(s) 'github_identity_token', valid inputs are
['claude_code_oauth_token', 'allowed_bots', 'extra_allowed_tools',
 'extra_prompt', 'model', 'review_depth', 'premortem']

What this does

Syncs claude-pr-review to documentation main (dc68b9c). Beyond
github_identity_token, the stale copy was missing the entire staged review
pipeline the current action shells out to — review_pipeline.py,
review-output.schema.json, and test_review_pipeline.py were never carried
over — plus incremental_model, the 🔄 Review in progress sticky comment, and
the semantic-stage turn budgets.

The action loads everything through $GITHUB_ACTION_PATH, so it relocates
unchanged; no consumer edits are needed. mega-agents and mega-e2e start honouring
their identity token the moment this merges.

Also adds .github/workflows/actions-test.yml. These actions ship to every repo
in the org at @main, so a merge here reaches all consumers with no consumer-side
review — documentation gated that with the pipeline unit tests on every PR, and
this repo had no CI at all. 77 tests, passing locally.

One intentional divergence from upstream

test_workflow_loads_actions_from_trusted_main asserts the repo's own
claude.yml consumes the actions via owner/repo@main rather than the local
./.github/actions path — the guard that stops a PR from modifying the action
that reviews it. That invariant belongs to a consumer repo; this repo is the
actions' home and has no claude.yml, so the test now skips when the file is
absent (and expects megaeth-labs/.github/... when one is added). It's the single
skip in the run.

Notes

• The README.md here is documentation's current version, with the Home note
  and the pr-lint section preserved and all megaeth-labs/documentation/...
  references repointed. Two upstream sentences describing documentation's own
  dogfooding were rewritten for this repo.
• Unrelated, spotted while checking: the migration commit message and README both
  describe a .github/workflows/pr-lint.yml reusable workflow, but only
  .github/actions/pr-lint/action.yml exists on main. The README's usage
  example is for the composite action and is correct; the workflow appears to have
  been dropped. Not touched here.
• documentation still carries its own copies, so it is now the fork that's ahead.
  Deleting them (and repointing documentation's claude.yml) is the natural
  follow-up once all consumers are migrated.