docs(cache): producer half of the depended-upon names contract (#11)

Co-authored-by: Claude <claude@gitdan.com>
Co-committed-by: Claude <claude@gitdan.com>

README.md

@@ -287,6 +287,36 @@ Lowering either here needs no coordination: the arbiter then only defers a
 reclamation this side would already have permitted, which costs disk rather
 than correctness.
 
+### Names this repo doesn't reclaim, but the arbiter depends on
+
+The five names above are leftovers — dead trees gitdan's arbiter finds and
+deletes. Four other dot-prefixed names change that arbiter's *behaviour*
+without ever being a leftover: it reads them to make a correctness decision
+and never reclaims them. They are a real cross-repo dependency too, on the
+name's spelling rather than on the tree's lifetime — gitdan's
+`DEPENDED-UPON NAMES CONTRACT` block (in `scripts/ci-cache-reclaim.sh`) is the
+canonical description; this is the producer's half for the two we own.
+
+| Name | Produced here by | Consequence of an unannounced rename |
+|---|---|---|
+| `.ci-lock-<id>` | `scripts/cache-lock.sh` (acquire/release) and `cache-lib.sh`'s `write_cache_lock()` | 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 |
+| `.cache-last-used` | `cargo-cache/action.yml`, stamped every run | The arbiter falls back to directory mtime — silently reordering its eviction order, and possibly failing to recognise the directory as a cache dir at all |
+
+Two more names are in gitdan's list — `.gitea-last-used` and `.ci-keep` — and
+neither is produced by anything in this repo, so there is no producer-side
+half to write here. `.gitea-last-used` is a naming convention individual
+repos used before adopting this shared action; nothing here writes it, and
+gitdan's script reads it only for compatibility with directories created
+under that older scheme. `.ci-keep` is a per-repo, hand-placed opt-out a
+consuming repo's own workflow drops directly into a cache directory it wants
+exempted from eviction — never something this action or its scripts write.
+
+**Adding a fifth depended-upon name counts as much as renaming one of the two
+above.** If a future change here makes gitdan's arbiter start depending on the
+spelling of some new dot-prefixed name — a name it reads for a decision but
+never reclaims — that is exactly this category, and it needs the matching
+`DEPEND_*` entry on gitdan's side before it ships, not after.
+
 ---
 
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