Say "low confidence" in the report, not just in the number - #397

v0.7.4 stopped a class carried by nothing but tool invocations from claiming diagnosis-level confidence — but the reports still printed it in the exact shape of a confident verdict:

Root cause: ruby_bundle_failure
Confidence: 0.3
Subsystem: Ruby dependency installation or test lifecycle

A maintainer reads "root cause" and goes to debug a Gemfile that is fine. The number dropped; the message did not.

What changes

Both human-facing renderers (--format text and --format markdown) add an explicit caveat when the confidence falls under LOW_CONFIDENCE_THRESHOLD (0.35 — one constant next to the existing report constants): that the verdict is a hint rather than a proven cause, the honest mechanism behind it (the signals only prove a tool ran, not that it failed), and the next useful step — the raw log, plus the existing CI failure fixture issue template if the real cause is something no class covers.

After, on rails/rails run 29648807728 (--format markdown):

Low confidence (0.3). Treat this as a hint, not a proven cause: the signals that carried this class only show that a tool ran, not that it failed, so the real failure may be something PatchRail does not recognize yet.

Read the raw log before changing anything — and if the cause is clear there, open a CI failure fixture issue with a sanitized log so PatchRail learns it.

Scope

• The guard keys on confidence alone, never on a class or an ecosystem: 0.35 sits between the invocation-only cap (0.3) and the lowest confidence any rule that actually watched something fail can earn (0.53).
• unknown declines at 0.15 and already has its own "help improve PatchRail" message, so it is excluded rather than told the same thing twice.
• Presentation only. No change to classify.py, to the class, the confidence, the exit code, or the --format json payload the GitHub Action consumes. A high-confidence report is byte-identical to before.

Verification

• New tests: text and markdown over the committed tests/data/realworld/rails-29648807728.log (the real 0.3 case), the JSON contract asserted clean, and springboot-29780604983.log asserted to keep its 0.89 verdict unqualified in both renderers.
• ci benchmark examples/ci-triage → 223/223, top-1 1.0 (unchanged).
• Full suite 1008 passed on Python 3.12; ruff check and ruff format --check clean.