fix(#22): add structured output instructions to code agent

The code agent was missing structured output guidance that the fix
and review agents already have. Every code agent run failed harness
validation with "output/code-result.json not found" because the
agent never knew it needed to produce the file.

Changes:
- agents/code.md: Add "Structured output" section (modeled on
  agents/fix.md) between "Constraints" and "Failure handling",
  documenting the MUST requirement to write the output JSON and
  validate it with fullsend-check-output.
- skills/code-implementation/SKILL.md: Add MANDATORY language and
  fullsend-check-output validation to the existing output-writing
  block in step 3. Add new step 11 ("Verify structured output")
  as a final check before exit with a recovery path if the file
  is missing.

Note: No test suite or lint command exists in this markdown-only
repo. Changes were verified by manual review of the diff.

Closes #22

Author: fullsend-ai-coder[bot]

agents/code.md

@@ -92,6 +92,36 @@ the review agent — if the triage was wrong, your code will fail review.
 - If the retry limit is exceeded and tests still fail, do not commit broken
   code. Stop. The post-script reports the failure.
 
+## Structured output
+
+You MUST produce a JSON file at `$FULLSEND_OUTPUT_DIR/$FULLSEND_OUTPUT_FILE`
+(typically `code-result.json`) that documents the target branch for PR
+creation. The post-script reads this file to create the PR against the
+correct branch. Without this file, the harness validation loop will reject
+the run.
+
+Write this file early — during step 3 of the skill, after determining
+the target branch — so it is available even if the agent hits a timeout
+or error later.
+
+The file must contain at minimum:
+
+```json
+{
+  "target_branch": "<branch-name>"
+}
+```
+
+After writing the file, validate it:
+
+```bash
+fullsend-check-output "${FULLSEND_OUTPUT_DIR}/${FULLSEND_OUTPUT_FILE}"
+```
+
+If validation fails, read the error output, fix the JSON file, and
+re-run the check. If it still fails after 3 attempts, write the best
+JSON you have and exit.
+
 ## Failure handling
 
 Secret scanning is **non-negotiable**. The `scan-secrets` helper runs before

skills/code-implementation/SKILL.md

@@ -186,8 +186,9 @@ use that branch. Otherwise, determine the repo's default branch:
 git rev-parse --abbrev-ref origin/HEAD | cut -d/ -f2
 ```
 
-Write your chosen branch to the structured output file so the post-script
-knows which branch to target the PR against:
+**MANDATORY: Write the structured output file.** You MUST write this file
+so the post-script knows which branch to target the PR against. Without
+this file, the harness validation loop will reject the run.
 
 ```bash
 mkdir -p "${FULLSEND_OUTPUT_DIR}"
@@ -198,6 +199,14 @@ cat > "${FULLSEND_OUTPUT_DIR}/${FULLSEND_OUTPUT_FILE}" <<RESULT
 RESULT
 ```
 
+Validate the output immediately after writing:
+
+```bash
+fullsend-check-output "${FULLSEND_OUTPUT_DIR}/${FULLSEND_OUTPUT_FILE}"
+```
+
+If validation fails, fix the JSON and re-run the check.
+
 Write this output early (during planning, after determining the target
 branch) so it is available even if the agent hits a timeout or error later.
 The post-script validates this against the repo's allowed branches.
@@ -739,6 +748,46 @@ message. The post-script runs an authoritative pre-commit on the runner.
 **Do not push the branch.** The post-script handles pushing, PR creation,
 and failure reporting.
 
+### 11. Verify structured output
+
+Before exiting, verify the structured output file exists and is valid.
+This is a **MANDATORY** final check — even if the commit succeeded, the
+harness validation loop will reject the run without this file.
+
+```bash
+echo "::notice::STEP 11: Verify structured output"
+```
+
+Check that the file exists:
+
+```bash
+test -f "${FULLSEND_OUTPUT_DIR}/${FULLSEND_OUTPUT_FILE}" \
+  && echo "Output file exists" \
+  || echo "ERROR: Output file missing"
+```
+
+If the file is missing (e.g., step 3 was interrupted before writing it),
+write it now:
+
+```bash
+mkdir -p "${FULLSEND_OUTPUT_DIR}"
+cat > "${FULLSEND_OUTPUT_DIR}/${FULLSEND_OUTPUT_FILE}" <<RESULT
+{
+  "target_branch": "<branch-name>"
+}
+RESULT
+```
+
+Validate the output:
+
+```bash
+fullsend-check-output "${FULLSEND_OUTPUT_DIR}/${FULLSEND_OUTPUT_FILE}"
+```
+
+If validation fails, read the error output, fix the JSON file, and
+re-run the check. If it still fails after 3 attempts, write the best
+JSON you have and exit.
+
 ## Partial work
 
 If you hit a token limit or context window boundary before completing the