Add Angr-backed CFG/FCG graph export to executable tool

[Copilot]

This issue asks for graph-based extraction (especially FCG) through docker-packing-box, similar to existing CFG support, and calls for an Angr-based extension in pbox.core.executable.
This PR adds a minimal graph export path for both CFG and FCG, exposed directly in the executable CLI.

• Core executable graph support

  • Added Executable.fcg (cached) to expose Angr’s function call graph from the knowledge base after CFG population.
  • Added Executable.graph(...) to export either graph type with a unified API:
    • graph_type: cfg | fcg
    • graph_format: dot | graphml
    • output: target path

• CLI surface (src/files/tools/executable)

  • Added graph subcommand to export graphs from a target executable.
  • New options:
    • --graph-type {cfg,fcg}
    • --format {dot,graphml}
    • --output <path>
  • Included a concrete FCG usage example in tool examples.

• Focused test coverage

  • Extended tests/executable.bats with a targeted FCG export test.
  • Verifies DOT output file creation and basic graph syntax (digraph).

executable graph /bin/ls \
  --graph-type fcg \
  --format graphml \
  --output /tmp/ls.fcg.graphml