feat(observability): write MCP task metrics

[canoo]

Summary

• Add richer MCP task-log metadata: task_type, model_provider, route_band, cost_usd, byte counts, input hash, and RFC3339 timestamp.
• Add a SQLite writer for new MCP task rows at ~/.config/nexus/logs/observability.sqlite when the Node runtime supports node:sqlite.
• Keep JSONL writes for compatibility with older runtimes and the current TUI fallback path.
• Update the TUI Task Log summary to show route-band counts and cloud-equivalent/local-cost totals.
• Extend task-log summary tests for route-band and cost math.
• Mark the first observability-schema migration steps as done in docs.

Issue Links

Refs #9, #10, #11.

Validation

• docker run --rm -v "${PWD}:/repo" -w /repo golang:1.26.2 gofmt -w tools/tui/main.go tools/tui/main_test.go
• docker run --rm -v "${PWD}:/repo" -w /repo/tools/tui golang:1.26.2 go test ./...
• node --check tools/mcp/server.mjs
• git diff --check -- tools/mcp/server.mjs tools/tui/main.go tools/tui/main_test.go docs/observability-schema.md

Notes

This is the first v0.2.0 observability implementation slice. It writes SQLite when available and keeps JSONL during the migration. The next slice should make the TUI prefer SQLite and fall back to JSONL.