feat(graph): surface unindexed source files instead of leaving them silent

[abhinav-phi]

Resolves #163.

Problem

EXTENSION_MAP covers ten extensions, and every discovery path is built from it, so a file with any other extension is invisible everywhere. The failure mode is silence:

• a repo with no supported files builds a graph of 0 nodes with exit code 0, and mex doctor shows four green ticks;
• a mixed repo (any Svelte, Vue or Astro frontend) indexes one of three source files, reports indexedFiles: 1 with no warning, and an agent asking about a symbol that exists gets the same TARGET_NOT_FOUND a typo gets.

Approach

Count the complement at the one seam every consumer already shares. corpus-policy.ts gains unindexedExtensionHistogram: a walk with the same glob options and the same ignore list as source discovery (so a repository's own graph.ignore shapes the answer), keeping only files whose extension is recognized source but no extractor claims it.

The recognized set is checked through isSupportedSourceFile against the live extension map, so an extension stops being reported the moment an extractor lands for it — the list cannot go stale.

Reporting is bounded and best-effort by contract: a file-scan cap with an explicit truncated flag, a capped histogram, and a walk that can never fail the command it decorates.

What changes

• mex graph: prints Not indexed: N source file(s) have extensions no extractor handles: .go (2), ... after the build summary; --json gains unindexedSources (total, byExtension, truncated), absent when the histogram finds nothing, so existing consumers are unchanged.
• mex graph query / mex impact: TARGET_NOT_FOUND records gain coverage context (filesIndexed, unindexedSources) — but only when it changes the record's meaning (nothing indexed, or unindexed source files exist). Ordinary misses in a healthy repository keep their exact prior shape.
• mex doctor: a Coverage line plus a next-steps hint when recognized source files are unindexable.

Verification

• New focused tests: coverage-histogram.test.ts (recognition, noise exclusion, graph.ignore handling, truncation, entry cap).
• cli-agent.test.ts: two new tests — TARGET_NOT_FOUND stays bare on a fully covered repository, and names unindexed sources on a mixed TS/Svelte/Go repo.
• Local smoke test reproducing the issue's Repro B: mixed TS/Svelte/Go repo now reports filesIndexed: 1 + unindexedSources: {total: 2, byExtension: {".go": 1, ".svelte": 1}} on both the build output and the TARGET_NOT_FOUND record; a clean repo's output is byte-identical to before.
• npm run typecheck, npm run build pass locally; targeted suites green. (The pre-existing Windows-local failures in symlink/SHM tests reproduce identically on clean main and are unrelated.)

No identity, reconciliation, schema, or drift-semantics changes are included.

[abhinav-phi]

Rebase note: this branch now sits on current main. The only code conflict was in cli-agent.ts where #193 added the configDriftRecords(session) loop just before the TARGET_NOT_FOUND emit — both kept, in that order: the config-drift records write first, then the error record with the coverage context. The CHANGELOG.md entry moved under ## [Unreleased] → ### Added (the old section this landed in was released as 0.8.1).

CI: the first run on the rebased head failed only in test/telemetry-delivery.test.ts (expected 357.77 to be less than 150) — a flush-grace timing assertion in the 0.8.1 telemetry tests, unrelated to this diff (it touches corpus-policy, cli-graph, cli-agent's error records, doctor and test/cli-smoke only). Re-triggered clean: all checks now pass (check 22.22.0/24, hub-browser, release-performance, storage-portability macos+windows), and the abstention golden from #189/#193's timeline of changes coexists with the coverage-context tests here.

All green — ready for review.