fieldtest: build the artefact from the current tree before running

A field test that runs a stale pre-built binary inverts its own
purpose — it reports the previously-shipped state as current, producing
false positives on already-fixed bugs and masking newly-introduced
ones. This bit an AILang raw-buf field test: the agent ran a
target/release binary built before a fix landed, reported a fixed bug as
still-broken, and masked a real check-clean/build-crash defect until
orchestrator re-verification against a fresh build.

Phase 2 now opens with a mandatory build-from-HEAD step (run
commands.build, or invoke the tool through the build system rather than
a path to a pre-existing artefact; build the release profile if the
examples invoke a release binary), and the agent records which build the
run exercised.

fieldtest/agents/fieldtester.md

@@ -120,6 +120,18 @@ Each phase completes before the next starts.
 
 ### Phase 2 — Implement each example as a downstream consumer
 
+**Before running any example, build the project's artefact from the
+current working tree.** A field test that runs a stale pre-built binary
+silently inverts its own purpose: it reports the *previously shipped*
+state as current, producing false positives on already-fixed bugs and
+masking newly-introduced ones. Run the build command from
+`commands.build` (or build the specific consumer binary the examples
+invoke) and confirm it succeeds before Phase 2 step 1; if the examples
+invoke a release binary, build the release profile, not just debug. When
+in doubt, invoke the tool through the build system (e.g. `cargo run`)
+rather than a path to a pre-existing artefact, so HEAD is always what
+runs. Record which binary/build the run exercised.
+
 For each example, in this order:
 
 1. Draft the program / task in the project's canonical