ci(gitea): gate scripts/ with shellcheck and the selftests

This repo had eight scripts, six selftest suites and nothing that ran any of
them. It is a composite action consumed at `@v1` — a moving tag — by three
repos' CI, so a bad edit here reaches zemyna, emowheel and lublub at once and
is discovered by whichever of them builds next.

One job: `shellcheck -x --source-path=scripts scripts/*.sh`, then
`bash scripts/selftest.sh`. `-x` follows the `. cache-lib.sh` every script
sources, which is where most of the logic being checked lives; without it
shellcheck reports SC1091 and analyses each file with a hole in it.

The full suite, not `--fast`: `hardlink-clone-selftest.sh` and
`restore-mtimes-selftest.sh` are the two that drive a real Cargo rather than a
fixture, and selftest.sh's own header says `--fast` is for iterating, not for
signing off a change. So the job installs a stable toolchain. The scratch
workspaces they build use path dependencies only, so nothing reaches
crates.io, and the job references no credentials at all.

Modelled on daniel/gitdan's workflow, including the two clauses of the
draft-skip guard (`github.event_name != 'pull_request' ||
!github.event.pull_request.draft` — the first is what stops the second from
also skipping pushes to `main`, where there is no `pull_request` context) and
the per-commit concurrency group for `push`.

No `container.volumes:` entry, deliberately: the suites want a cold target dir
every run, since "did this rebuild?" is exactly what restore-mtimes-selftest
asks. This job takes no share of the shared CI cache disk budget.

Turning the gate on surfaced three pre-existing findings, all fixed here
rather than suppressed or waived:

- `restore-mtimes-selftest.sh` SC2038: `find | xargs touch` -> `-print0 | xargs -0`.
- `hardlink-clone-selftest.sh` SC2295: `${f#$base_fix/}` -> `${f#"$base_fix"/}`.
- `cache-lib.sh` SC2016: a per-line disable with the rationale. The single
  quotes are load-bearing — the program is for the inner shell, where `$f` is
  its loop variable, `$rc` its accumulator and `$$` its pid — so this is the
  documented-false-positive case, not a stopgap.

.gitea/workflows/ci.yaml

@@ -0,0 +1,83 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+    # Spelled out only to keep `ready_for_review` in the list — naming any type
+    # replaces the whole default set, so the other three have to be restated.
+    # It is inert on this instance (draft state here is the `WIP:` title
+    # prefix, so un-drafting is a title edit and raises no
+    # `ready_for_review` action) and costs nothing.
+    #
+    # The consequence, which is the part that bites: the `if:` guard below is
+    # evaluated when a run is CREATED, and un-drafting creates no run. A PR
+    # opened as a draft keeps its skip decision until something else produces
+    # one. Push an empty commit after un-WIP'ing.
+    types: [opened, synchronize, reopened, ready_for_review]
+
+# gitdan-ci runs four repos' CI on two capacity slots, and the compiler-backed
+# suites below are multi-minute. A superseded run costs a slot in front of
+# somebody's build, so drop it.
+#
+# `push` groups on `github.sha` rather than `github.ref`: a constant per-branch
+# group is what let this Gitea (1.26.0) cancel two of daniel/gitdan's merge
+# runs outright while `cancel-in-progress` was gated away from `push` entirely
+# — see the long note in that repo's ci.yaml for the evidence. Giving every
+# commit its own group leaves that behaviour nothing to act on.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event_name == 'pull_request' && github.ref || github.sha }}
+  cancel-in-progress: true
+
+jobs:
+  selftest:
+    name: shellcheck + selftests
+    # Two clauses, both load-bearing. The second skips draft PRs: Gitea sets
+    # draft:true when the title starts with `WIP:`, so work-in-progress pushes
+    # cost the shared runner nothing until the PR is un-WIP'd. The first is
+    # what keeps that from also skipping pushes to `main` — a `push` event has
+    # no `pull_request` context, so `github.event.pull_request.draft` is empty
+    # there and the negation alone would be unreliable. Never drop it.
+    if: ${{ github.event_name != 'pull_request' || !github.event.pull_request.draft }}
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+    # Deliberately no `container.volumes:` entry, unlike every repo that
+    # CONSUMES this action. The suites below build throwaway workspaces under
+    # `mktemp -d` and want a cold target dir every time — a persistent cache
+    # would make "did this run rebuild?" unanswerable, which is the question
+    # restore-mtimes-selftest.sh exists to ask. So this job takes no share of
+    # the shared CI cache disk budget.
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v4
+
+      - name: Install shellcheck
+        uses: taiki-e/install-action@v2
+        with:
+          tool: shellcheck
+
+      # `hardlink-clone-selftest.sh` and `restore-mtimes-selftest.sh` drive a
+      # real Cargo against a real scratch workspace — they are the only things
+      # here that verify the hardlink-aliasing and mtime-freshness behaviour
+      # against the compiler rather than against a fixture, and selftest.sh's
+      # own header says `--fast` is for iterating, not for signing off a
+      # change. So CI installs a toolchain and runs the full set.
+      #
+      # The scratch workspaces use path dependencies only, so nothing here
+      # reaches crates.io.
+      - name: Install Rust toolchain
+        uses: dtolnay/rust-toolchain@stable
+
+      # Every script, including the suites themselves. `-x` follows the
+      # `. cache-lib.sh` each one sources, which is where most of the logic
+      # being checked actually lives; without it shellcheck reports SC1091 and
+      # analyses each file with a hole in it.
+      - name: shellcheck
+        run: shellcheck -x --source-path=scripts scripts/*.sh
+
+      # One command, not six: selftest.sh is the entry point a developer runs,
+      # so a suite added there is gated here without a matching edit in this
+      # file.
+      - name: Selftests
+        run: bash scripts/selftest.sh