docs: rename 'fake-agent' → 'sample-agent' (matches colibri test rename)

Harness-neutral, lighter wording for the optional local test-double agent
(colibri-test-agent), matching the colibri-side fake→sample rename. Only the
two references that named it 'fake-agent' (build.cfg comment, AGENTS.md
staging note); the unrelated /tmp/fake-usb example path in FIRSTBOOT.md is a
different context and left as-is.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

AGENTS.md

@@ -144,7 +144,7 @@ the ISO does **not** build Rust while the image is mounted.
 - Production-image preflight requires the core Colibri set:
   `colibri-daemon`, `colibri`, and `colibri-mcp`. `colibri-tui` is optional in
   staging code but desired for this USB target and should be verified alongside
-  the required binaries. Validation images can add the local fake-agent helper
+  the required binaries. Validation images can add the local sample-agent helper
   with `COLIBRI_STAGE_TEST_AGENT=YES`.
 
 **Invariant:** preserve `/home/clawdie/ai/colibri/target/release` until the ISO

build.cfg

@@ -115,7 +115,7 @@ COLIBRI_COST_MODE="${COLIBRI_COST_MODE:-smart}"
 # build host compiles it: (cd $ZOT_REPO && git checkout $ZOT_VERSION &&
 #   ZOT_BUILD_VERSION="${ZOT_VERSION:-v0.2.42}" && VERSION="${ZOT_BUILD_VERSION#v}" make build)
 COLIBRI_STAGE_AGENT="${COLIBRI_STAGE_AGENT:-YES}"
-# Development-only local fake-agent binary. Auto mode includes it for dev
+# Development-only local sample-agent binary. Auto mode includes it for dev
 # builds and omits it from release/prod images; override with YES/NO for
 # explicit validation builds.
 COLIBRI_STAGE_TEST_AGENT="${COLIBRI_STAGE_TEST_AGENT:-auto}"