chore(repo): ignore macOS AppleDouble metadata files (#4720)

## Thinking Path

> - Paperclip is the open source app people use to manage AI agents for
work
> - Contributors clone this repository and work in it on macOS
> - macOS writes AppleDouble sidecar files named `._<filename>` next to
real files
> - The repository ignores `.DS_Store`, but it does not ignore these
sidecars
> - `git status` then lists the sidecars together with real untracked
files
> - The noise hides real changes during development and review
> - This pull request adds the standard macOS sidecar patterns to
`.gitignore`
> - The benefit is a clean `git status` for every macOS contributor

## Linked Issues or Issue Description

No existing issue covers this. The problem is described below, in the
format of
`.github/ISSUE_TEMPLATE/enhancement.yml`.

**What existing behavior does this improve?**

The root `.gitignore` file, and the `git status` output that developers
see.

**Subsystem affected**

Cross-cutting (multiple of the above) — repository tooling, not
application code.

**Current behavior**

The root `.gitignore` ignores `.DS_Store`. It does not ignore the
AppleDouble
sidecar files that macOS writes. macOS creates a file named
`._<filename>` next
to a regular file when the volume cannot store extended attributes. This
happens
on exFAT and NTFS volumes, on FUSE mounts, and on network shares. Git
shows
every one of these files as untracked. The author reports approximately
58,000
such files in one local checkout. Real untracked files, such as new
scaffold
directories, are lost in that list.

**Proposed behavior**

`.gitignore` ignores `._*`, `.AppleDouble`, and `.LSOverride`. `git
status` then
shows only real files. These three patterns are the macOS set from
GitHub's
gitignore collection:
https://github.com/github/gitignore/blob/main/Global/macOS.gitignore

**Reason and benefit**

Developers lose time when they read `git status` output that contains
thousands
of operating system files. They can also commit a sidecar file by
accident. The
repository already suppresses these same files in its own code: `ssh.ts`
sets
`COPYFILE_DISABLE=1` to stop `bsdtar` from writing them, and the Daytona
file-sync path excludes them from tarballs. This change applies the same
rule
to git.

**Breaking changes**

None. No tracked file matches the new patterns. Git continues to track
any file
that is already tracked, because `.gitignore` applies to untracked files
only.

## What Changed

- Added `._*`, `.AppleDouble`, and `.LSOverride` to the root
`.gitignore`, below
  the existing `.DS_Store` entry.

## Verification

- `git status` no longer lists `._*` files after the change.
- `git ls-files | grep -E '^\._|\.AppleDouble|\.LSOverride'` returns no
output.
  No tracked file becomes ignored.
- `.DS_Store` behavior does not change.

## Risks

Low risk. This is a configuration change with no runtime effect. A file
that
starts with `._` becomes ignored, so a contributor who needs such a file
must
add it with `git add -f`. Git refuses to add an ignored path without
that flag,
so the effect is visible and not silent. No such file exists in the
repository.

Note for contributors who already have these files in a working tree:
the ignore
rule does not delete them. Remove them with:
`find . -name '._*' -not -path './node_modules/*' -not -path './.git/*'
-delete`

## Model Used

- Provider: Anthropic Claude, through Claude Code.
- The original description recorded only "Generated with Claude Code".
It did not
  record an exact model ID.
- This description was rewritten to the repository template by Claude
Opus 5
(1M context, extended thinking, tool use), on behalf of the maintainers.
The
  code in this pull request is unchanged.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have searched GitHub for duplicate or related PRs and linked
them above (no duplicates found)
- [x] I have either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have not referenced internal/instance-local Paperclip issues or
links
- [x] My branch name describes the change
(`chore/gitignore-appledouble`) and contains no internal ticket id
- [ ] I have run tests locally and they pass — not applicable, no code
is executed by this change
- [x] I have added or updated tests where applicable (not applicable —
`.gitignore` only)
- [x] I have updated relevant documentation to reflect my changes (none
required)
- [x] I have considered and documented any risks above
- [x] All Paperclip CI gates are green
- [ ] Greptile is 5/5 with no open P2s — pending re-review of this
updated description
- [x] I will address all Greptile and reviewer comments before
requesting merge

Co-authored-by: User <user@example.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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