feat: add cross-host telemetry coverage

[pacphi]

Summary

Implements the post-PR175 decision for issue #170 and adds the requested dashboard/API plumbing.

• Adds additive sourceHealth.<host>.diagnostics.common coverage diagnostics.
• Adds explicit supported, unsupported, and unavailable capability states for Claude, Codex transcript evidence, and OpenCode.
• Preserves existing status, reason, Codex diagnostics, and cache schema compatibility.
• Keeps Codex command/file/MCP/collaboration items as bounded diagnostic evidence; does not recast them as tool calls or host-specific scorecard totals.
• Adds a visible Usage telemetry coverage panel with old-API fallback and no fabricated zeroes.
• Bounds unknown wire-kind diagnostics to 32 names plus overflow volume.
• Documents the contract and the August 24, 2026 public API cross-check.

Public-contract validation

• Claude Code hooks and monitoring: https://code.claude.com/docs/en/hooks and https://code.claude.com/docs/en/monitoring-usage
• Codex app-server protocol: https://github.com/openai/codex/blob/main/codex-rs/app-server/README.md
• OpenCode SDK/message model: https://github.com/anomalyco/opencode/blob/dev/packages/web/src/content/docs/sdk.mdx and https://github.com/anomalyco/opencode/blob/dev/packages/opencode/src/session/message.ts

The public surfaces are richer and not wire-compatible with one another. A future normalized activity taxonomy still needs decisions on category semantics, nested-agent behavior, deduplication, and privacy before adding comparable execution/file/MCP/collaboration counters.

Validation

• pnpm run check
• 129 focused telemetry/usage/citation tests
• Dashboard integration: 72 passed, 0 failed

PR #175 remains the unmerged decision record; this branch implements the accepted boundary.