docs(cache): write down the producer half of the depended-upon names contract

daniel/gitdan's ci-cache-reclaim.sh hard-codes four dot-prefixed names it
reads to make correctness decisions but never reclaims as leftovers —
.ci-lock-*, .cache-last-used, .gitea-last-used, .ci-keep — none of which
carried a matching note here (gitdan#32, the follow-up shape to #28/#30
that #7/PR #8 covered for the leftover names specifically).

Of the four, this repo actually produces two:

- .ci-lock-<id>, written by scripts/cache-lock.sh (acquire/release) and
  cache-lib.sh's write_cache_lock(). Documents the contract at
  cache-lock.sh's header, the site someone renaming the marker would
  most likely be editing.
- .cache-last-used, stamped every run by cargo-cache/action.yml. Documents
  the contract at the exact line that writes it.

The other two are read by gitdan's script but produced by nothing in this
repo: .gitea-last-used is a legacy naming convention individual repos
used before adopting the shared cargo-cache action (nothing here writes
it today), and .ci-keep is a per-repo, hand-placed opt-out any consuming
repo's own workflow may drop directly into a cache directory, with no
single owner. Both get a paragraph in README.md's new subsection
explaining why no producer-side counterpart exists for them, rather than
inventing an owner this repo doesn't have.

README.md's "Scratch names in a cache root are a cross-repo contract"
section gains a new subsection, "Names this repo doesn't reclaim, but
the arbiter depends on", covering all four and pointing at gitdan's
DEPENDED-UPON NAMES CONTRACT block as the canonical description.

No behaviour change: comments and docs only.

Ref: gitdan#32. Consumer-side counterpart: daniel/gitdan (this branch's
sibling PR).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MJHKJFJVUjnvVsemVBnvrd

scripts/cache-lock.sh

@@ -14,6 +14,19 @@
 # until STALE_LOCK_SECONDS, after which it is treated as abandoned by a job
 # the runner killed before it reached its own release step. Honouring a lock
 # forever would let one crashed job pin a directory permanently.
+#
+# `.ci-lock-<id>` IS A CROSS-REPO CONTRACT NAME with daniel/gitdan's
+# `scripts/ci-cache-reclaim.sh`: that host-level arbiter reads this same
+# marker (its `leftover_is_locked()`) to decide whether a leftover it is about
+# to reclaim is still held open by a live job. `cache-lib.sh`'s
+# `write_cache_lock()` writes the identical shape independently of this
+# script. Rename or reshape this marker here without a matching change on
+# gitdan's side and the arbiter's lock check silently stops matching — a live
+# job's staging tree loses its liveness guard and becomes an ordinary reclaim
+# candidate while still in use. See the "Scratch names in a cache root" section
+# in README.md, and daniel/gitdan's `DEPENDED-UPON NAMES CONTRACT` block in
+# `scripts/ci-cache-reclaim.sh`, for the full arrangement — this repo's half
+# is: don't rename `.ci-lock-` without telling that script.
 set -euo pipefail
 
 MODE="${1:?usage: cache-lock.sh acquire|release <dir> <lock-id>}"