Support .stashignore and .gitignore for file filtering

[rupertsworld]

Problem

Stash currently hardcodes its file filtering logic in listTrackedFiles():

if (entry.name.startsWith(".")) {
  continue;
}

This has two issues:

1. No user-configurable ignore rules. There's no way to exclude specific files or directories (e.g. logs/, scratchpad/, node_modules/) from syncing. The only option is to move them out of the stash directory entirely.

2. All dot-files/directories are unconditionally skipped. This is a reasonable default, but it means you can never sync a dot-file even if you want to (e.g. .gitignore, .editorconfig, .claude/). There's no opt-in mechanism.

Proposal

.stashignore

Add support for a .stashignore file at the stash root, using gitignore-style glob patterns:

# Ephemeral / local-only
logs/
scratchpad/
inbox/
read-later/
*.local
*.local.*

# Large / binary
node_modules/
*.mp4

This file itself would be tracked and synced, so ignore rules are consistent across environments.

.gitignore support

If a .gitignore exists in the stash root, respect it as well. .stashignore rules would be additive on top of .gitignore. This is useful when a stash is also a git repo, so you don't have to duplicate ignore rules.

Dot-file handling

Change the default behavior: instead of blanket-skipping all dot-files, only skip .stash/ (stash internals) by default. Let .stashignore handle everything else. Users who want the current behavior can add .* to their .stashignore.

This is a breaking change in the sense that dot-files that were previously invisible will start syncing. A migration path:

• On stash init, seed .stashignore with .* and !.stashignore to preserve current behavior
• Existing stashes get a warning on first sync after upgrade, suggesting they review their dot-files

Implementation notes

The ignore npm package handles gitignore-style pattern matching and is well-tested. It would slot into listTrackedFiles() as a filter predicate, replacing the startsWith(".") check.

Alternatives considered

• Config in .stash/config.local.json: Doesn't sync across environments, and JSON arrays of globs are harder to read/edit than a gitignore-style file.
• Only .gitignore: Not all stashes are git repos, and stash-specific ignores (like *.local) shouldn't pollute .gitignore.