Document the discovery API in docs/api.md

Full client-facing reference: auth, all routes with schemas and curl
examples, selection/cooldown semantics, caveats. README and
architecture.md link to it.

Co-Authored-By: Claude <noreply@anthropic.com>

docs/plans-executions/2026-08-11-2152-discovery-api-docs.md

@@ -0,0 +1,38 @@
+# Execution: Discovery API documentation
+
+Plan: [2026-08-11-2152-discovery-api-docs.md](../plans/2026-08-11-2152-discovery-api-docs.md)
+
+- [x] Step 1 — Write `docs/api.md` full API reference
+- [x] Step 2 — Add pointers in README and architecture.md
+
+## Step 1 — docs/api.md
+
+Wrote the full reference: base URL (in-cluster FQDN + port-forward), bearer
+auth, error envelope, configuration table, all five routes with schemas,
+status codes and curl examples, the selection/cooldown semantics section,
+an end-to-end curl walkthrough, and the in-memory/single-replica caveats.
+All facts were taken from the code, not from memory of prior docs.
+
+Worth noting: the doc explicitly calls out two things no earlier doc
+stated for clients — that a report **without** a target on a targetless
+lease creates a *global* cooldown (blocking the proxy for everyone), and
+that `rate_limited` and `banned` currently behave identically. Both came
+straight from `internal/lease/store.go` and are easy to trip over.
+
+## Step 2 — Pointers + verification
+
+Added one-line links to the new doc in README's quickstart (above the curl
+block) and in `docs/architecture.md` §7. Verified the documented behavior
+against the tree rather than trusting the write-up:
+
+```sh
+go build ./... && go test -short ./internal/discovery/ ./internal/lease/
+```
+
+Both pass; a grep of `server_test.go` confirmed every documented status
+code and error code (`invalid_ttl`, `invalid_query`, `invalid_result`,
+`no_match`, `unknown_lease`, 201/204/401/404/409) is asserted by tests.
+
+Worth noting: CHANGELOG entry deliberately deferred until the user
+confirms the docs read well, per the CHANGELOG convention's
+"once the user confirms it works" clause.