Consolidate Diffly UX and verdict improvements

[VIVAAN-DHAWAN]

Summary

This PR consolidates the setup/tutorial UX and judging improvements into one reviewable change set.

Interactive UX

• diffly with no arguments opens the guided PR wizard
• diffly setup teaches the complete workflow and can launch the wizard
• diffly help, diffly doctor, and diffly version provide discoverability and diagnostics
• interactive PR review supports arrow navigation, section toggles, and focused rendering
• diffly is the primary executable while diffly-cli remains a compatibility alias

Judging system

• canonical verdicts are PASS, QUARANTINE, and BLOCK
• failed checks and security-sensitive production changes block
• dependency, database, coverage, pending, and unknown-check risks quarantine
• clean, confirmed-successful reviews pass
• older integrations receive legacy_verdict: SHIP when the canonical verdict is PASS
• coverage heuristics avoid documentation, fixtures, generated files, JSON, and installer false positives

GitHub Action

• gracefully handles GitHub 404s when PR comment publishing is unavailable
• carries forward the useful reliability fixes from superseded PR Fix audit findings: Correctness, Policy, API and Installer improvements #4

PR #8 was closed as superseded; its setup wizard/tutorial code is included in this PR because the branch is now based directly on main.
PR #4 was previously closed after its Action check completed with a GitHub comment API 404.

Verification

• python3 -m compileall -q src tests action scripts passed
• git diff --check passed
• full pytest remains unavailable in this environment because pytest and runtime dependencies are not installed

[github-actions]

Diffly verdict: QUARANTINE

#9 · 5 files · 63 lines changed · checks: PENDING

Risk flags: NO_TEST_COVERAGE (medium), CHECKS_UNKNOWN (medium)

Risk flags and reasoning

• NO_TEST_COVERAGE (medium): Changed production files have no obvious neighboring or repository test coverage. — README.md; src/diffly_cli/cli.py
• CHECKS_UNKNOWN (medium): Required status checks are missing, pending, or unavailable. — pending
• QUARANTINE because at least one changed production file lacks obvious test coverage.
• QUARANTINE because the affected pull request does not have a confirmed passing check result.

Blast-radius summary

File	Change	Symbols	Direct callers	Tests
README.md	+3/-3	<top-level changes>	—	—
action/post_comment.py	+14/-6	<top-level changes>	—	tests/test_action.py
src/diffly_cli/cli.py	+3/-2	<top-level changes>	—	—
src/diffly_cli/triage.py	+18/-4	_added_dependency_names	—	tests/test_triage.py, tests/test_triage.py
tests/test_triage.py	+9/-1	<top-level changes>	—	—

Deterministic triage is authoritative; any optional LLM explanation cannot change the verdict.