feat: Gitea Actions compatibility test suite

A self-validating test suite that proves Gitea's implementations of
GitHub-style actions behave like GitHub's. Push to any Gitea instance
with a registered runner — on push or manual dispatch, it runs 10
independent test workflows covering checkout, artifacts, caching,
language setup, expression contexts, workflow commands, services,
composite/reusable workflows, control flow, and known divergences.

Components:
- lib/assert.sh — self-asserting shell helper library (assert_eq,
  assert_contains, assert_exists, assert_not_exists, assert_match,
  sha256_of) with verified negative fail path
- 10 reusable workflows (on: workflow_call):
  01-checkout, 02-artifacts, 03-cache, 04-setup-runtime,
  05-contexts, 06-workflow-commands, 07-services,
  08-composite-reusable, 09-control-flow, 10-gitea-divergences
- 00-suite-runner.yml — sole entry (on: push + workflow_dispatch):
  probe gate (Gitea >= 1.22.0) -> c1-c10 parallel callers
  (c5/c8 forward secrets: inherit for act_runner #125) ->
  aggregate (if: always, skipped-as-failure)
- Fixtures: LFS 5MB binary, git tag v1-test, 4 language lockfiles
  (npm/go/pip/maven), sparse-cone layout
- Makefile: make lint (actionlint static gate)
- README: prerequisites, per-test expectations, correction table,
  manual eyeball checklist for UI-only behaviors

lib/composite-greet/action.yml

@@ -0,0 +1,24 @@
+name: 'Composite Greet'
+description: 'Local composite action used by C8 to verify input/output round-trip'
+
+inputs:
+  who:
+    description: 'Who to greet'
+    required: true
+
+outputs:
+  greeting:
+    description: 'The greeting string'
+    value: ${{ steps.greet.outputs.greeting }}
+
+runs:
+  using: composite
+  steps:
+    - id: greet
+      shell: bash
+      env:
+        WHO: ${{ inputs.who }}
+      run: |
+        # shellcheck disable=SC2154
+        greeting="Hello, ${WHO}!"
+        printf 'greeting=%s\n' "$greeting" >> "$GITHUB_OUTPUT"