Feature: `.zenignore` to exclude paths from vault indexing

[yfernandes]

Summary

Add support for a .zenignore file at the vault root that lets users exclude
directories and files from indexing, search, and the folder tree — using
gitignore-compatible syntax.

Motivation

Some vaults contain more than notes. When a vault directory also holds
programming artifacts (e.g. node_modules, __pycache__, build output), every
one of those files is currently walked, read for metadata, and surfaced in the
sidebar. This inflates load times and clutters the tree with content that isn't
a note.

Concretely, a single project folder in my vault indexes 2385 items, of which
2209 come from node_modules — so ~93% of the indexing cost for that folder
is pure noise.

Current behavior

Vault content is walked in several independent places, all sharing the same
skip rules (resolveDirDescent + entry.name.startsWith('.')):

• listNotes() — the note-metadata index (main load-time cost)
• listFolders() — the sidebar folder tree
• collectBuiltinSearchCandidates() — JS full-text search fallback
• collectRipgrepSearchCandidates() — ripgrep full-text search
• VaultWatcher — the chokidar ignored predicate for live updates

There is already precedent for path exclusion: VaultWatcher hardcodes
node_modules in its ignore predicate. That skip is not honored by the
load-time walks, and it isn't user-configurable. .zenignore generalizes it.

Proposed behavior

• On scan, read <vault-root>/.zenignore (if present) and build a
  gitignore-compatible matcher.
• Apply the matcher in each walk so ignored directories are skipped before
  descending into them — that's where the load-time win comes from.
• Honor it in the watcher and both search backends so ignored paths don't
  reappear via live updates or search results.
• Keep the existing built-in node_modules default so current behavior
  doesn't regress; .zenignore adds to it rather than replacing it.

Open questions

• Scope: root-only .zenignore vs. nested per-directory files (like git).
  Root-only is much simpler and covers the primary use case; suggest starting
  there.
• Syntax source: adopt exact gitignore semantics (least surprising) via a
  small dependency rather than hand-rolling glob matching?

Notes on implementation

The startsWith('.') skip + descent check is currently duplicated across the
walks. Extracting that shared skip logic into one helper first would let the
ignore matcher be threaded through a single place, keeping the feature small and
the diff easy to review.

Happy to implement this if the approach sounds good — I'd wait for agreement on scope (root-only vs. nested) before starting.