docs: v0.2 features

README, examples, and a comment update covering v0.2's four features:

- README: new Caching section (key semantics incl. requestedModel +
  endpoint, hit/miss header, count-requests-free-tokens, group override,
  redis-required) and Admin section (enable flag, admin:true users,
  browser key-entry flow, sessionStorage, what's exposed/redacted,
  poll-counts-against-quota, unauthenticated-shell). Retry and Image/audio
  sections already existed from T3/T4; left as-is. Config reference gains
  cache/admin rows at the top level, GroupConfig.cache, UserConfig.admin,
  and full CacheConfig/AdminConfig tables. Known limitations gains
  images/audio-not-cost-accounted and cache-shares-entries-across-users.
- examples/kubernetes.yaml: adds cache{enabled:true,ttl:5m} (this example
  already configures redis) and admin{enabled:false} with a comment on
  enabling it, plus an inline admin:true user gated behind that flag.
  Round-trips through New() cleanly (verified with a throwaway test).
- examples/docker-compose.yml: adds retry, cache (left disabled — this
  minimal demo runs no Redis), and admin (left disabled, with a comment
  on enabling it). Round-trips through New() cleanly (verified the same
  way).
- admin.go: pluginVersion's doc comment now says to keep it in sync with
  the release tag.

admin.go

@@ -12,7 +12,8 @@ import (
 // read-only in GET /admin/api/overview (spec §4, v0.2). Bumped alongside
 // the module's own release process — not read from anywhere else in the
 // package, since Yaegi interpretation has no build-info mechanism to pull
-// it from automatically.
+// it from automatically. Keep in sync with the release tag: bump this
+// alongside `git tag`, not before or after it.
 const pluginVersion = "0.2.0"
 
 // The three routes the read-only admin dashboard registers (spec §4,