refactor(runtime): compact generated agent context

[chubes4]

Closes #934

Summary

• replace enumerated DMC command lists with live --help discovery pointers
• replace the default full workspace inventory with a bounded authoritative-command summary
• retain explicit full inventory mode for sites that require embedded detail
• remove the obsolete reflection-based command introspector
• enforce a compact default inventory size budget

How to test

1. Run php tests/smoke-agents-md-sections.php.
2. Run php tests/smoke-agents-md-workspace-freshness.php.
3. Run git diff --check origin/main...HEAD.

Size evidence

The source diff removes 306 lines and adds 71. The default fixture inventory renders at 492 bytes with a test-enforced 700-byte ceiling; detailed mode remains available through datamachine_code_workspace_inventory_mode=full.

Compatibility

The default inventory mode changes from embedded per-repository detail to summary. Sites needing the old detailed snapshot can opt into full; live workspace commands remain authoritative.