feat(cli): add cora trace and cora arch subcommands

[ajianaz]

cora trace + cora arch subcommands

What

Two new CLI commands that leverage the graph database (schema v3) built in Phase 2.

cora trace <symbol>

• BFS path tracing through the codebase
• --direction outgoing (default): follows what the symbol calls
• --direction incoming: follows what calls the symbol
• --depth N: max hops (default 3)
• --json: structured output
• Uses edges table (typed: CALLS, IMPORTS, IMPLEMENTS...) with call_graph fallback

cora arch

• Architecture overview: modules by symbol count
• Edge type distribution (CALLS, IMPORTS, etc.)
• --json: structured output

Testing

• 3 new unit tests: trace outgoing, trace incoming, arch overview
• 708 total tests pass, 0 clippy warnings
• Manual test on cora-code repo: 1695 symbols, 5358 CALLS edges across 109 files

Example

$ cora arch
  MODULE  SYMBOLS  EDGE TYPES
  src          1663           7
  tests          25           1
  EDGE DISTRIBUTION
  CALLS              5358

$ cora trace open_global_index --depth 2
  depth 1
      data_dir::graph_db_path [CALLS] src/index/mod.rs:31
      schema::run_migrations  [CALLS] src/index/mod.rs:35
  depth 2
      ...