Add per-area debt budgets / SLOs with CI gating

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Summary

Add per-area debt budgets / SLOs so teams can cap how much debt a directory or package is allowed to carry and gate CI when an area regresses past its budget.

Motivation

Baselines freeze all current debt globally; quality gates apply repo-wide severity policy. Neither lets a team say "the new src/payments module must stay under 20 issues" or "src/legacy may not get worse." Per-area budgets give teams a burn-down target and protect freshly-cleaned areas from backsliding.

Proposed behavior

• New config block:

  {
    "budgets": {
      "src/payments": { "maxIssues": 20, "maxHigh": 0 },
      "src/legacy":   { "maxIssues": 250 }
    }
  }

• During scan, aggregate issues per matching path glob and compare against the budget.
• A budget breach fails the gate (respecting existing --gate / exit-code policy) with a clear per-area message and remaining headroom.
• debtlens scan --budget-report prints a per-area table (used / budget / headroom) without failing.

Implementation surface

• Extend src/config/schema.ts, src/config/loadConfig.ts, src/config/validateConfig.ts, and src/config/mergeConfig.ts for the budgets block; reuse src/config/workspaces.ts glob matching.
• New src/core/budgets.ts to evaluate budgets against ScanResult; integrate with src/core/gatePresets.ts / src/core/severity.ts exit-code policy.
• Reporter support for the budget table.
• Schema drift test (tests/config/schema.test.ts) + budget evaluation tests.

Acceptance criteria

• budgets validated by schema and merged with documented precedence.
• Breaches fail the gate with per-area detail; --budget-report is non-failing.
• Glob matching consistent with existing package/workspace resolution.
• Docs + CHANGELOG updated.
• npm run test:all passes.

Difficulty: medium.