Add convention-based traceability fallback (#135)

[Dimwiddle]

Description

Implement convention-based traceability inference for discovery/status flows.

What changed

• Added src/specleft/discovery/traceability.py with TraceabilityLink and infer_traceability(discovered, specs).
• Integrated convention fallback into specleft status when decorator indexing has no match.
• Added match_kind metadata to status entries and rendered ✓ (convention) in table output.
• Extended generate_draft_specs(...) to optionally emit linked_tests frontmatter blocks for matched scenarios.
• Added tests in tests/discovery/test_traceability.py, extended tests/commands/test_status.py, and extended tests/discovery/test_spec_writer.py.
• Updated features/feature-spec-discovery.md with story/acceptance coverage for issue Discovery Phase 2: convention-based test traceability #135.

Type of Change

• New feature
• Tests
• Documentation

Testing

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

Related Issues

Closes #135