feat(extensions): support .extensionignore to exclude files during install (#1781)

* feat(extensions): support .extensionignore to exclude files during install

Add .extensionignore support so extension authors can exclude files and
folders from being copied when users run 'specify extension add'.

The file uses glob-style patterns (one per line), supports comments (#),
blank lines, trailing-slash directory patterns, and relative path matching.
The .extensionignore file itself is always excluded from the copy.

- Add _load_extensionignore() to ExtensionManager
- Integrate ignore function into shutil.copytree in install_from_directory
- Document .extensionignore in EXTENSION-DEVELOPMENT-GUIDE.md
- Add 6 tests covering all pattern matching scenarios
- Bump version to 0.1.14

* fix(extensions): use pathspec for gitignore-compatible .extensionignore matching

Replace fnmatch with pathspec.GitIgnoreSpec to get proper .gitignore
semantics where * does not cross directory boundaries. This addresses
review feedback on #1781.

Changes:
- Switch from fnmatch to pathspec>=0.12.0 (GitIgnoreSpec.from_lines)
- Normalize backslashes in patterns for cross-platform compatibility
- Distinguish directories from files for trailing-slash patterns
- Update docs to accurately describe supported pattern semantics
- Add edge-case tests: .., absolute paths, empty file, backslashes,
  * vs ** boundary behavior, and ! negation
- Move changelog entry to [Unreleased] section

CHANGELOG.md

@@ -7,6 +7,12 @@ Recent changes to the Specify CLI and templates are documented here.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Added
+
+- feat(extensions): support `.extensionignore` to exclude files/folders during `specify extension add` (#1781)
+
 ## [0.2.0] - 2026-03-09
 
 ### Changed
@@ -43,7 +49,6 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - fix: release-trigger uses release branch + PR instead of direct push to main (#1733)
 - fix: Split release process to sync pyproject.toml version with git tags (#1732)
 
-
 ## [0.1.14] - 2026-03-09
 
 ### Added