feat(overview): plain-text monthly usage overview command

[iamtoruk]

What

Adds codeburn overview, a copy-pasteable plain-text usage report.

• Defaults to the current month. --from/--to filters any range, plus --provider, --project, and --exclude.
• --no-color for plain output; colored cyan section headings otherwise (also stripped automatically when the output is piped).
• Sections: Totals, By tool, Top models, Highest-value days, Top projects, a per-day Daily table, By activity, and Tools, rendered as clean bordered tables.

Notes

• Reads from the existing in-memory session aggregation (the same data the dashboard and menubar use), so it is unaffected by the report --format json validity issue (unescaped control characters and a NaN/Infinity-style literal) found while building this. That serializer bug is worth fixing separately.
• Display-only polish lives in the command: cost gains thousands separators and tokens roll up to a B unit, via local wrappers that do not change the shared formatCost/formatTokens, so other commands' output and tests are untouched.
• Project rows use the path basename (e.g. HackRome, eywa) rather than the sanitized full path.

Verification

• tsc clean; full suite green (1242) including 3 new tests in tests/overview.test.ts.
• Verified against real local data for the current month.

Example

CodeBurn  June 2026

Totals
  Cost       $2,795.10
  Tokens     3.49B   in 23.9M / out 20.2M / cache-w 72.5M / cache-r 3.38B
  Calls      14,755   sessions 753
  Cache hit  99.3%

By tool
┌──────────┬───────────┬────────┬───────┐
│ Tool     │      Cost │ Tokens │ Share │
├──────────┼───────────┼────────┼───────┤
│ claude   │ $2,662.37 │  3.34B │   95% │
│ codex    │   $119.12 │ 128.1M │    4% │
└──────────┴───────────┴────────┴───────┘