Make diffly interactive, colorful, and easier to use

[VIVAAN-DHAWAN]

Summary

• add the shorter diffly executable while preserving diffly-cli as a compatibility alias
• add diffly help, diffly version, and diffly doctor
• add diffly pr ... --interactive with arrow-key navigation, space-to-toggle sections, Enter to render, and q to quit
• improve terminal output with Rich panels, colors, tables, and actionable API errors
• update the installer and README for the new command and keyboard workflow
• add parser regression coverage for the new commands and flag

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

#6 · 5 files · 155 lines changed · checks: PENDING

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

Risk flags and reasoning

• NEW_DEPENDENCY (medium): Adds or changes a dependency manifest or lockfile. — pyproject.toml
• NO_TEST_COVERAGE (medium): Changed production files have no obvious neighboring or repository test coverage. — README.md; install.sh; pyproject.toml; src/diffly_cli/cli.py
• CHECKS_UNKNOWN (medium): Required status checks are missing, pending, or unavailable. — pending
• QUARANTINE because dependency changes expand the supply-chain and runtime surface.
• 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	+15/-10	<top-level changes>	—	—
install.sh	+5/-3	<top-level changes>	—	—
pyproject.toml	+1/-0	<top-level changes>	—	—
src/diffly_cli/cli.py	+109/-3	_section_lines	_section_lines	—
tests/test_regressions.py	+9/-0	<top-level changes>	—	—

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