Add an 'Observing your miner' doc section to packages/gittensory-miner/docs

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Context

None of the markdown files under packages/gittensory-miner/docs/*.md currently document how to observe a running miner -- specifically, how to point Grafana at the miner's local SQLite ledgers using the new datasource/dashboard work. This is distinct from the general self-host operations runbook (#4875, maintainer-only), which does not call out this miner-specific observability subsection.

Dependencies

None -- independently shippable. This is a documentation-only addition; it can be written and merged regardless of where C36/C37/C39 stand, though it becomes more useful once the miner-usage.json dashboard and datasource it describes exist.

Requirements

1. Add a new documentation section -- either as a new file (e.g. packages/gittensory-miner/docs/observability.md) or a new section within an existing doc, matching however this docs folder currently organizes multi-topic guides.
2. Document how to configure Grafana to point at the miner's local SQLite ledger(s) using the new datasource.
3. Document how to load/find the corresponding dashboard (e.g. miner-usage.json) once the datasource is configured.
4. Include concrete, copy-pasteable steps (not just prose) -- e.g. the datasource connection string/path pattern, any plugin required to query SQLite from Grafana, and where the dashboard JSON lives.
5. If a new file is created, add a link to it from coding-agent-driver.md's table of contents so it's discoverable from the existing entry point contributors already read.
6. Keep scope to observability/Grafana setup only -- do not duplicate content already covered by the general self-host ops runbook; cross-reference it instead if relevant.

Deliverables / Acceptance Criteria

• New 'Observing your miner' section or file added under packages/gittensory-miner/docs/
• Section documents SQLite-ledger-to-Grafana datasource setup with concrete steps
• Section documents locating/loading the miner dashboard
• coding-agent-driver.md's table of contents updated with a link to the new section/file
• No content duplicated verbatim from the maintainer-only ops runbook (Write an operational runbook #4875) -- link out instead

Test Coverage Requirements

This PR must ship with full test coverage for every changed line and branch -- the repo's Codecov patch gate requires 99%+ coverage and the house standard is to aim for 100%, including both sides of every conditional/nullish-coalescing branch introduced. This issue is documentation-only, so test coverage in the strict Codecov sense does not apply since src/** is the only patch-gated path; instead: (1) if the repo runs a markdown-link-checker or docs-TOC-consistency check in CI, verify the new file/section and the updated coding-agent-driver.md link both pass it, (2) an invariant check that the new doc's TOC entry actually resolves to the new section/file (no dead anchor), and (3) no regression test applies since this is net-new documentation, not a bug fix.

Expected Outcome

A contributor or operator setting up a miner for the first time can follow one doc section to get Grafana showing their miner's activity from its local SQLite ledgers, instead of having to reverse-engineer the datasource/dashboard wiring themselves.

Links & Resources

• packages/gittensory-miner/docs/ (folder to add the new section/file to)
• packages/gittensory-miner/docs/coding-agent-driver.md (table of contents to update)
• See Write an operational runbook #4875 (general self-host ops runbook, maintainer-only, related but distinct)
• Theme: Dashboard/observability parity

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JSO-3727