fix(phased-execution): resume failed session on retry, surface errors, thinking/compaction indicators

- retries now resume the failed executor's session (pre/post session
  tracking replaces the broken mtime-vs-ref check)
- report recovery from the session file when the JSON stream loses its
  tail (child signaled mid-flush)
- progress.mjs: thinking indicators (◐ thinking… / ◑ thought for Ns),
  compaction (⧉ …) and provider auto-retry lines; trims trailing
  whitespace so no blank lines after LLM text; exits 1 on truncated
  stream, model error, or missing final text (pi --mode json always
  exits 0 even on errors)
- explicit ✗ ERROR lines + exit codes: 0 ok, 1 phase failed, 130/143
  interrupted (INT/TERM traps; post-pipeline check covers the case
  where bash suppresses the INT trap after a job dies from SIGINT)
- PIPESTATUS captured in a single statement (any following command
  resets it)
- skills dir: .gitignore README.md so pi's skill scanner (which honors
  .gitignore) stops warning 'description is required'

.gitignore

@@ -0,0 +1,5 @@
+# pi treats any root-level .md file in a skills directory as a skill and
+# warns "description is required" for non-skill markdown like this README.
+# pi's skill scanner honors .gitignore files, so ignoring README.md here
+# keeps it out of skill discovery (and out of git).
+README.md