Add 63 unit tests for agent_scaffold edge cases and scanner other_phases

- agent_scaffold_coverage_tests.rs: 30 tests covering CallGraphBuilder and
  FunctionLookup edge cases (empty inputs, disconnected components, recursive
  functions, diamond call patterns, multi-language indexing, non-existent
  targets, duplicate function names across files)
- scanner_other_phases_tests.rs: 33 tests exercising all 20 phase handlers in
  other_phases.rs via the public run_phase dispatcher (Indexing, Semgrep,
  CweRouting, RuleSynthesis, ConfidenceScoring, etc.)
- mod.rs: declare both new test modules

Unit test count: 2944 -> 3007. CI gate: fmt clean, clippy clean, all tests pass.

Author: Mte90