feat(cli): add convenience filter flags --ext, --max-depth, --min-size, --max-size

[gregpriday]

Summary

Adds four convenience CLI flags that make common file-filtering tasks fast and intuitive, without requiring glob patterns or profile edits.

Closes #28

Changes Made

New CLI flags (bin/copytree.js)

• --ext <extensions> — filter by file extensions, comma-separated (e.g. .js,.ts,.tsx or js,ts)
• --max-depth <n> — limit traversal depth (0 = root files only, 1 = one level deep, etc.)
• --min-size <size> — exclude files smaller than a human-readable threshold (e.g. 1KB, 500B)
• --max-size <size> — exclude files larger than a human-readable threshold (e.g. 10MB, 1GB)

Wiring (src/commands/copy.js)

• Added parseExtensions() — normalises comma-separated extensions to lowercase with leading dots; throws CommandError for empty/degenerate input (e.g. ',,,)
• Added parseSizeOption() — wraps existing parseSize() from helpers.js with a user-friendly CommandError
• buildProfileFromCliOptions() — converts the four new flags into profile.options fields
• setupPipelineStages() — forwards the new options to FileDiscoveryStage

Filtering logic (src/pipeline/stages/FileDiscoveryStage.js)

• Constructor stores extFilter, maxDepth, minSizeBytes, maxSizeBytes
• maxDepth is passed to both walkers via walkOptions
• Extension and size filters are applied as post-filters in the discovery loop — AND logic with existing --filter patterns
• Force-included files (--always / .copytreeinclude) bypass ext/size filters by design (separate fast-glob path)

Depth limiting (src/utils/ignoreWalker.js, src/utils/parallelWalker.js)

• walkWithIgnore — walk() now accepts a depth parameter (default 0); recursion is skipped when depth >= maxDepth
• walkParallel — BFS queue entries carry depth; subdirectories are only enqueued when depth < maxDepth
• Both walkers use identical semantics; parity is verified by a cross-walker test

Tests

• New tests/unit/pipeline/stages/FileDiscoveryStage.filters.test.js — 18 tests covering all four filters and their combinations
• Extended tests/unit/utils/ignoreWalker.test.js — 4 maxDepth cases (0, 1, 2, unlimited)
• Extended tests/unit/utils/parallelWalker.test.js — 5 maxDepth cases including sequential/parallel parity check

Usage Examples

# Only JavaScript/TypeScript files
copytree --ext .js,.ts

# Limit depth to 2 levels
copytree --max-depth 2

# Files between 1KB and 100KB
copytree --min-size 1KB --max-size 100KB

# Combined: Python files, 3 levels deep, under 50KB
copytree --ext .py --max-depth 3 --max-size 50KB

# Combined with existing --filter (AND logic)
copytree --filter "src/**" --ext .ts