feat: add token usage tracking and cost visibility

[spboyer]

Closes #453

Summary

Adds token usage tracking and cost visibility to Squad. Users can now see how many tokens each agent consumes and optionally set budget limits.

What's New

squad cost CLI command

• View token usage from orchestration logs
• Filter by agent or view all historical data
• Formatted table output with per-agent breakdown

BudgetDefinition config

• Optional perAgentSpawn and perSession token limits
• Configurable warning threshold (warnAt)
• defineBudget() builder with full validation (rejects NaN, Infinity, negatives)

Orchestration log enhancement

• New Token usage row in log entry template
• Scribe prompt updated to record usage data from agent responses

Tests & Docs

• 25 unit tests (CostTracker, defineBudget validation, log parsing)
• 5 CLI integration tests
• Feature documentation page

Review Notes

• FIDO review caught NaN/Infinity validation gap in defineBudget() - fixed with Number.isFinite() checks
• Existing CostTracker class and OTel metrics were already in the SDK - this PR wires them into the user-facing flow
• Orchestration log change is backward compatible (missing usage rows return null)

[spboyer]

CI Status

CI is blocked by a pre-existing build issue (#455).

The CLI package depends on @bradygaster/squad-sdk version 0.8.25 from the npm registry (a fixed version, not a workspace link). The published v0.8.25 doesn't include the roles module that was added in PR #369. This causes tsc to fail with 'has no exported member listRoles'.

This affects ALL PRs targeting main that build the CLI, not just this one. Fix tracked in #455.

This PR's changes (cost tracking) are correct and tested - 173 tests pass locally (25 cost + 148 roles). The CI failure is unrelated to cost tracking.

[spboyer]

cc @bradygaster — PR is ready but CI is blocked by #455 (roles module not in published SDK). Cost tracking changes are tested and correct, just need the upstream build fix to land first.