feat: emit <no-hash> for zero-match entries instead of failing

[maastrich]

Summary

When a config entry's glob matches zero files, hashup now emits the sentinel <no-hash> for that entry and continues processing the rest instead of aborting the whole run.

app           48adf62a70c2645d0fc15ee3060973245af5dc30a542372791a7e1f05eaeacf6
visual-tests  <no-hash>
worker        0c4b8d9f…

In --json mode: { "hash": "<no-hash>", "files": [] }. The sentinel is also exported as NO_HASH from src/cli/run-config-mode.ts for programmatic callers.

Motivation

Monorepo configs regularly glob files that aren't always present — a package that doesn't have visual tests yet, a feature-flagged directory, a generated dir that hasn't been seeded. Previously a single zero-match entry aborted the invocation; now it's a local, addressable signal downstream tooling can detect without gate-keeping the whole run.

Test plan

• Updated tests/cli/run-modes.test.ts:
  • Mixed config ({ none: "src/*.missing", real: "src/a.ts" }) succeeds; none emits <no-hash>, real emits a real digest
  • --json + --files for a zero-match entry emits { hash: "<no-hash>", files: [] }
• vp test — 121/121 passing
• vp check clean
• End-to-end smoke in both plain and JSON CLI modes

Docs

docs/guide/cli.md — replaced the "zero-match is an error" paragraph with the new sentinel behavior and a sample output block.

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[github-actions]

📖 Docs preview: https://maastrich.github.io/hashup/branches/pr-21/