feat(init): idempotent + --force/--out

[ali90h]

feat(init): idempotent + --force/--out

What / Why

This PR implements the init command to create default devcontainer configurations with idempotent behavior, force overwrite capability, and custom output path support.

Key Features:

• Idempotent behavior: Won't overwrite existing files without --force flag (returns exit code 0)
• Force overwrite: --force flag enables overwriting existing devcontainer files
• Custom output paths: --out PATH supports writing to custom locations with automatic parent directory creation
• Atomic file writes: Uses temporary file + rename pattern for safe file operations
• Proper exit codes: 0=success/exists, 1=I/O errors, 2=misuse errors
• Default configuration: Python 3.11, Node 20, and Go 1.22 features with venv setup

Architecture:

• Pure env.py module with exception-based error handling (no prints, no exit codes)
• CLI layer in cli.py handles exit codes and user messages
• Comprehensive test coverage with unit, integration, and subprocess tests

Fixes #4

How I Tested

Ran the complete validation suite locally:

pre-commit clean && pre-commit run --all-files  # ✅ All hooks passed
pytest -q                                       # ✅ 52 tests passed
python -m black --check .                      # ✅ Formatting verified

Test Coverage:

• Default devcontainer configuration validation (Python 3.11, Node 20, Go 1.22)
• Idempotent behavior without --force flag
• Force overwrite functionality with --force
• Custom output paths with --out option
• Parent directory creation
• Error handling: invalid paths, permission denied, directory conflicts
• Exit code validation for all scenarios
• CLI integration with mocked and subprocess testing
• Atomic file write operations

Output Snippets

First time creation:

$ autorepro init
Wrote devcontainer to ./devcontainer.json

Idempotent behavior (file already exists):

$ autorepro init
devcontainer.json already exists at ./devcontainer.json.
Use --force to overwrite or --out <path> to write elsewhere.

Force overwrite existing file:

$ autorepro init --force
Wrote devcontainer to /path/to/dev/devcontainer.json

Custom output location:

$ autorepro init --out dev/devcontainer.json
Wrote devcontainer to /path/to/dev/devcontainer.json

Acceptance

• ✅ Implements exact message wording as specified in Issue # T-002 — init: idempotent + --force / --out #4
• ✅ Idempotent behavior with proper exit code 0 when file exists
• ✅ Force overwrite functionality with --force flag
• ✅ Custom output paths with automatic directory creation
• ✅ Atomic file writes using temp file + rename pattern
• ✅ Proper exit codes: 0=success/exists, 1=I/O errors, 2=misuse
• ✅ Default devcontainer includes Python 3.11, Node 20, Go 1.22
• ✅ Comprehensive test coverage (23 new tests)
• ✅ CI: should be green on this PR