docs: per-module READMEs (run vs library) + root navigation hub

Every subproject now has README.md:
- 3 runnable modules (:standalone, :agentik-cli, :agentik-tui):
  quickstart, env table, parameters, known limits
- 11 library modules: what it is, which problem solves, how to
  wire it in, where versions live

Root README.md is the navigation hub (Quickstart, Modules table,
publish + CI/CD notes).

Also: ci.yml prunes the :memory-vector -x excludes now that
text-embedding-kmp artifacts are published to caffeine.

518 tests green.

Verified publish pipeline: :proto:publish to caffeine produces
pom.module + per-target klibs + sources for all 9 KMP targets.

🤖 Generated with [opencode]

storage-core/src/commonTest/kotlin/pw/binom/agentik/storage/PayloadTest.kt

@@ -59,7 +59,7 @@ class PayloadTest {
     }
 
     @Test
-    fun `legacy plain-array payload still decodes (backward compat)`() {
+    fun `legacy plain-array payload still decodes backward compat`() {
         val legacy = "[" +
             """{"type":"text","body":"old message"}""" +
             "]"