feat(plan): subscription plan tracking with usage progress bar

Adds `codeburn plan set <id>` to configure a subscription plan (Claude Pro,
Claude Max, Cursor Pro, or custom). When set, the Overview panel renders
an API-equivalent progress bar against subscription price with a
projected month-end cost.

Closes the loudest demand signal on the repo: issue #11 ("Subscription
vs API Use") from two independent voices, plus the routing-decision use
case raised in #12.

- src/config.ts: extends CodeburnConfig with Plan, adds readPlan/savePlan/clearPlan
- src/plans.ts: presets (claude-pro $20, claude-max $200, cursor-pro $20)
- src/plan-usage.ts: getPlanUsage, resetDay-aware period math (1-28),
  median-of-7-day-trailing projection
- src/cli.ts: `codeburn plan [show|set|reset]` subcommand, plan wired
  into JSON outputs for report/today/month/status (only when active)
- src/dashboard.tsx: Plan row in Overview, color-coded (green under 80%,
  orange near, red over), with days-until-reset
- README.md: Plans section with honest framing (API-equivalent vs
  subscription price, not token allowance)
- tests/plan-usage.test.ts, tests/plans.test.ts, tests/cli-plan.test.ts:
  period math, presets, CLI round-trip

Resets respect resetDay across month boundaries. Uses median daily spend
(not mean) so one huge day doesn't distort the month-end projection.

Fixes #11

README.md

@@ -166,6 +166,22 @@ The currency setting applies everywhere: dashboard, status bar, menu bar widget,
 
 The menu bar widget includes a currency picker with 17 common currencies. For any currency not listed, use the CLI command above.
 
+## Plans (subscription tracking)
+
+If you're on Claude Pro, Claude Max, or Cursor Pro, set your plan so the dashboard shows subscription-relative usage:
+
+```bash
+codeburn plan set claude-max                                  # $200/month
+codeburn plan set claude-pro                                  # $20/month
+codeburn plan set cursor-pro                                  # $20/month
+codeburn plan set custom --monthly-usd 150 --provider claude # custom
+codeburn plan set none                                        # disable plan view
+codeburn plan                                                 # show current
+codeburn plan reset                                           # remove plan config
+```
+
+The progress bar shows API-equivalent cost vs subscription price. Presets use publicly stated plan prices (as of April 2026); they do not model exact token allowances, because vendors do not publish precise consumer-plan limits.
+
 ## Menu Bar
 
 <img src="https://cdn.jsdelivr.net/gh/getagentseal/codeburn@main/assets/menubar-0.8.0.png" alt="CodeBurn macOS menubar app" width="420" />