feat(graph): vault-wide statistics when sourcePath omitted (closes #132)

[aaronsb]

Summary

Closes #132. `graph.statistics` previously threw `Source path is required` when called without `sourcePath`, blocking the baseline use case (vault-health snapshots, dashboards). Users had to pick an arbitrary seed and reason about its neighborhood, which isn't representative of the vault.

When `sourcePath` is omitted, the operation now returns a new `vaultStatistics` shape (per the issue's proposal):

Field	Meaning
`totalNotes`	Count of `.md` files in the vault
`totalLinks`	Sum of resolved link occurrences — Obsidian semantics: A→B referenced 3× counts as 3
`orphanCount`	Singletons (no resolved links in either direction)
`averageDegree`	`2 * totalLinks / totalNotes`
`largestComponentSize`	Biggest connected subgraph (treated as undirected)
`isolatedClusters`	Total connected-component count, inclusive of singletons (non-trivial = `isolatedClusters - orphanCount`)

Per-node statistics behavior is unchanged when `sourcePath` is provided.

Implementation

• `GraphTraversal.getVaultStatistics()` — one O(V+E) pass over the `resolvedLinks` adjacency, plus BFS for components. Treats the graph as undirected for component analysis (a link from A to B means A and B are in the same component) while keeping link counts directed (matches how `metadataCache.resolvedLinks` exposes them).
• `GraphSearchTool.getStatistics()` — branches on `!sourcePath` and returns the vault-wide shape with a different message + workflow hints. Throw-on-missing path removed.
• `GraphSearchResult.vaultStatistics?` — added alongside existing `statistics?` rather than overloading, since the shapes are genuinely different and a discriminated union would just push the question to consumers.

Tests

`tests/graph-statistics-vault-wide.test.ts` (7 cases):

• vault-wide path with a known 3-component topology (A→B,C / D→E / F orphan) → asserts exact `vaultStatistics` values
• per-node path with `sourcePath` still returns the old `statistics` shape
• repeated occurrences count toward `totalLinks` (Obsidian semantics)
• empty vault: no divide-by-zero, all zeros returned
• non-`.md` files excluded from `totalNotes`
• bidirectional links collapse to one undirected component

Test plan

• `npm run build` — type-checks clean
• `npm test` — 319/319 across 27 suites (was 312 before, +7 new)
• `npm run lint` — no new errors (5 pre-existing warnings unchanged)
• Live MCP smoke against the running vault — needs this release shipped first to update the local plugin

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