session: Harness-Session-Variable schliesst die Luecke im Session-Id-Fallback (Telemetrie-Join, Iteration-Budget-Gate); SIGPIPE-Nebenbefund im Emitter behoben

Files changed:
- CHANGES.md
- EVALS.md
- INSTALL.md
- VERSION
- instructions/session-setup.md
- tools/CONTRACT.md
- tools/chemenu/cli.py
- tools/chemenu/commands/doctor.py
- tools/chemenu/commands/run_budget.py
- tools/chemenu/session.py
- tools/chemenu/telemetry/writer.py
- tools/chemenu/tests/conftest.py
- tools/chemenu/tests/test_cli.py
- tools/chemenu/tests/test_run_budget.py
- tools/chemenu/tests/test_telemetry_emit.py

EVALS.md

@@ -57,7 +57,21 @@ flowchart TD
 - **Hooks enrich.** They add the tool calls the repo layer cannot see: file reads, greps,
   shell commands, prompts.
 
-Everything joins on `WIKITOOL_SESSION_ID`.
+Everything joins on one session id, resolved the same way by every source that has to pick
+one - see `chemenu.session`. The chain is `WIKITOOL_SESSION_ID`, then a harness's own session
+variable where one is registered (`chemenu.session.HARNESS_ENV_VARS` - Claude Code's
+`CLAUDE_CODE_SESSION_ID` today), then the parent process id. The middle step exists because
+the last one does not survive a harness that runs every tool call in its own freshly
+initialised shell: `os.getppid()` is then a new "session" per call, and neither the join nor
+the Iteration Budget Gate below can see more than one or two calls of a real run. A harness
+only earns an entry in that chain once a live session has been observed setting the variable,
+confirmed to be the exact id its own hooks write elsewhere in a trace - a name that merely
+looks plausible would mis-key a session more quietly than the pid fallback it replaced.
+
+A trace hook that only *observes* tool calls (a `PreToolUse`/`PostToolUse`-style wiring) does
+not by itself fix a harness whose events carry a different id than `wikitool`'s own emitter -
+the two still would not join. Wiring such a hook is only worth doing once this fallback chain
+already keys both sides on the same id.
 
 ## The trace
 
@@ -68,7 +82,7 @@ is [tools/chemenu/telemetry/schema.py](tools/chemenu/telemetry/schema.py).
 |---|---|
 | `v` | Schema version |
 | `ts` | ISO-8601 UTC, microsecond precision |
-| `session_id` | The join key. `WIKITOOL_SESSION_ID`, else the parent process id |
+| `session_id` | The join key - `chemenu.session`'s fallback chain: `WIKITOOL_SESSION_ID`, else a registered harness variable, else the parent process id |
 | `pid`, `seq` | `seq` counts **within one process**. Sort a trace by `(ts, pid, seq)` |
 | `source` | `wikitool`, `runner`, or a harness name |
 | `event` | See below |