observability: AST linter check_event_names

[AbdelStark]

Problem

RFC-0013 INV-OBS-1/2 require every logged event name and every metric name to be a literal registered in EVENTS / METRICS. Without a linter, a typo like logger.info('training.step') vs 'training_step' slips through silently.

Solution

• tools/lint/check_event_names.py

  • registered_event_name: walks every logger.{debug,info,warn,warning,error}(...) call. If the first positional arg is a string literal, asserts it is in EVENTS. Dynamic event names (Name or f-string) are skipped — they're a runtime check.
  • registered_metric_name: same shape for counter.inc() / histogram.observe() / gauge.set(). Receiver-name heuristic restricts the check to obvious metric handles to avoid flagging unrelated .inc() calls.
  • Discovers both registries by AST-parsing observability.py — no runtime import.
  • Skips observability.py itself (the EventSpec(...) literals would otherwise look like raises).
  • Returns early on METRICS discovery if absent, so this check stays dormant until observability: implement metrics registry + Prometheus textfile exporter #25 ships.

• .github/workflows/lint-errors.yml renamed to lint and gains a check-event-names job alongside check-error-codes.

• tests/lint/test_check_event_names.py (19 cases): registry discovery; positive / negative event names; dynamic / f-string event names skipped; parametrized over all logger levels; metric check armed only when registry present; format spec; self-skip; no-args tolerated.

Validation

$ python -m pytest tests/ -q
224 passed in 0.69s

$ python -m tools.lint.check_event_names
$ echo $?
0

Caveats

• METRICS-check is structurally implemented but stays dormant until observability: implement metrics registry + Prometheus textfile exporter #25 lands the registry. When it does, no linter change is needed.
• False-positive rate on the metric receiver heuristic was the open trade-off. Decided to require a counter|histogram|gauge|metric substring in the receiver identifier — strict enough that ordinary box.inc(1) does not trigger.

Closes #27