fix(trust): ignore self-reports in ScoreAverageTrust

[owenamurray04]

Problem

ScoreAverageTrust.report() never checks evidence.reporter, so an agent can
file positive evidence about itself to inflate its own score, or dilute genuine
negative reports back toward neutral.

Change

• report() ignores evidence where evidence.reporter == agent. All other
  reports behave as before. Matches bonded_trust, which already skips
  self-reports when scoring.
• Documented in the method docstring.

Tests

• test_self_report_is_ignored: ten positive self-reports leave the agent at
  the never-rated default. Fails on main (score=1.0, sample_count=10).
• test_self_report_does_not_dilute_others: a self-boost cannot offset a
  genuine negative report. Fails on main (score=0.5, sample_count=2).

Verification

make ci-local passes all five gates (pytest: 1276 passed, 1 skipped).