Doc consistency: guide_ai_client.md, guide_app_controller.md, and
the error_handling styleguide now reference the new symbol name.
Also fixes two consistency issues in error_handling.md introduced by
the mechanical rename:
1. The 'Deprecation: send -> send_result' section (lines 623-642) was
rewritten as a 'Historical deprecation (added 2026-06-15, reverted
2026-06-16)' note that points to the relevant track specs.
2. Line 204 (the 'Current State Audit' summary for src/ai_client.py)
had a self-contradictory claim ('send() is the new public API;
send() is @deprecated') after the rename. Updated to describe
the canonical public API.
Historical archives (conductor/tracks/*/spec.md, conductor/tracks/*/plan.md,
docs/reports/*) are NOT modified - they document the 2026-06-15
public_api_migration decision and stay as historical record.
ed