Add discovery pipeline orchestrator foundations (#126)

[Dimwiddle]

Description

Implements discovery orchestration infrastructure for issue #126:

• Added DiscoveryConfig with from_pyproject()/default handling
• Added FrameworkDetector for Python + TS/JS framework detection
• Added immutable MinerContext
• Added DiscoveryPipeline with registration guards, language filtering, per-miner error isolation, and report aggregation
• Added build_default_pipeline() wiring (FileIndex, FrameworkDetector, MinerContext) and a minimal built-in language-agnostic miner (readme_overview) for baseline non-empty integration runs
• Added tests for config, framework detection, pipeline behavior, and integration on this repo
• Updated discovery feature spec scenarios/criteria for orchestration behaviors

Type of Change

• New feature
• Tests
• Documentation update

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that this feature works
• New and existing unit tests pass locally with my changes

Testing

• make lint-fix ✅
• make lint ⚠️ fails due pre-existing mypy errors in src/specleft/commands/features.py (lines 818, 1039, 1040), unrelated to this PR
• make test ✅ (473 passed)

Related Issues

Closes #126