fix: todos scan task ballooned to 2.5MB and analysis produced no output

The todos pre-scan walked .output/ (Nitro) and .vercel/ (Vercel) build
dirs, flagging 119 of 124 candidates inside minified bundles (single
lines up to 162KB). buildTodosScanTask embedded full candidate lines in
the task prompt, producing a 2.5MB prompt on freno-dev; the analysis
agent settled with ok:true + empty text + no findings.md, verify failed,
and resume re-ran the same oversized prompt and failed identically.

- scope: exclude .output/.vercel/.netlify (shared by all checks)
- todos: truncate candidate code at 160 chars in the prompt + fallback
- agent-runner: a session settling with no text and no observed message/
  tool events now fails the run loudly instead of reporting ok:true
- agent prompts: add the three dirs to each skip list
- tests: excluded-dir scan, prompt truncation, emptySessionError cases

agents/defensive-guards.md

@@ -87,8 +87,9 @@ noise the user cannot act on.
 
 ## Skip (directory names — never descend into)
 
-`.cache`, `.git`, `.hg`, `.idea`, `.next`, `.nuxt`, `.pygienium`, `.ralpi`,
-`.svelte-kit`, `.svn`, `.turbo`, `.vscode`, `__pycache__`, `build`, `coverage`,
+`.cache`, `.git`, `.hg`, `.idea`, `.netlify`, `.next`, `.nuxt`, `.output`,
+`.pygienium`, `.ralpi`, `.svelte-kit`, `.svn`, `.turbo`, `.vercel`, `.vscode`,
+`__pycache__`, `build`, `coverage`,
 `dist`, `node_modules`, `out`, `vendor`, `venv` (and `.venv`)
 
 ## Skip (file patterns)