fix: treat hierarchical tags as a tree in the tag-usage scanner

[rogerdigital]

Problem

Users who structure their vaults with hierarchical tags (#parent/child) saw parent tags flagged as "Low-usage tag" even while their children were actively used (#171). The scanner counted each tag as a flat string, so a parent written standalone only occasionally (below threshold) was reported as underused — a false positive by the check's own purpose: a parent with active descendants is a healthy branch root, not abandoned cruft.

The same flat counting also affected the watchlist: a watched parent that only ever appeared as #parent/child was reported as "does not appear in the vault", while Obsidian's tag:#parent search matches child tags.

Change

• Low-usage check: a tag is exempt when any descendant at any depth has a count ≥ the threshold (prefix-based, so grandparents are covered without recursion). Branches that are entirely below the threshold are still reported — that is the genuinely dead case the check exists for.
• Missing watched tag: a watched tag counts as present when any child tag appears in the vault.
• No new setting: this is a false-positive correction, not a preference — the new behavior only removes findings no reasonable workflow wants, and dead branches keep their signal. README updated to document the tree semantics.

Verification

• TDD: new tests observed failing first, then passing — parent exemption (reporter's case), grandparent exemption at depth, watched-parent presence, plus a control that dead branches are still flagged.
• Full local gate: npm run lint && npm run lint:obsidian-warnings && npm run build && npm test — 929 tests pass.
• Fingerprints of remaining findings are unchanged, so previously ignored findings stay ignored.

Fixes #171