Add per-project configuration via .claude/last30days.env, discovered by
walking up from cwd. Uses the same .env format as the existing global
config — no new parsers or formats.
Priority (highest wins):
1. Environment variables
2. .claude/last30days.env (per-project)
3. ~/.config/last30days/.env (global)
Also adds file permission checking — warns to stderr if config files
are readable by other users (should be chmod 600).
Includes tests for discovery, precedence, source tracking, and
permission warnings.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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