chore(agent)!: remove the no-op agent feature

It enabled nothing — the agent layer is always built — yet the README,
QUICKSTART and USE_CASES told people to pass it. Removed, with those
snippets fixed: they now depend on the git repository (nothing is on
crates.io, so `version = "2.0"` never resolved) and USE_CASES no longer
presents the `float16` feature as half-precision storage (that is
MemoryConfig::float16, on by default for new stores).

Breaking for anyone passing `features = ["agent"]`: drop it.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

CHANGELOG.md

@@ -3,6 +3,9 @@
 ## Unreleased
 
 ### Upgrade Notes
+- **Breaking:** `clawhdf5-agent`'s `agent` feature is removed. It enabled
+  nothing — the agent layer is always built — but the README and guides told
+  people to pass it; drop `agent` from `features = [...]`.
 - **`clawhdf5-migrate` now writes a real agent store.** Its output used to be
   a layout of its own (`/chunks`, `/sessions`, `/entities`, `/relations`, no
   `/meta`) that `HDF5Memory::open` rejected, so a migrated file could not be