Add specleft discover command and promote flow (#136)

[Dimwiddle]

Description

Add the new discovery command group for Phase 2 onboarding:

• specleft discover [OPTIONS] [PROJECT_ROOT]
• specleft discover promote [OPTIONS] [FEATURE_ID...]

What changed

• Added src/specleft/commands/discover.py with:
  • Full discovery execution via build_default_pipeline(...) + group_items(...) + generate_draft_specs(...)
  • --dry-run, --format json/table, --output-dir, repeatable --language, --specs-dir, and --pretty
  • JSON payload with features, totals, output_dir, dry_run, written paths, and errors
  • promote subcommand with --all, targeted feature IDs, --overwrite, and --dry-run
• Registered command in CLI wiring:
  • src/specleft/commands/__init__.py
  • src/specleft/cli/main.py
• Added CLI tests in tests/cli/test_discover.py covering dry-run, JSON output, output-dir override, promote all/single, overwrite/skip, and miner-error resilience.
• Updated docs and discovery feature scenarios:
  • docs/cli-reference.md
  • README.md
  • docs/getting-started.md
  • docs/SKILL.md
  • features/feature-spec-discovery.md

Type of Change

• New feature
• Tests
• Documentation

Testing

• make lint-fix && make lint && make test && make pre-commit (pass)

Related Issues

Closes #136