feat: per-agent spill controls

[thomwebb]

What

An individual agent can now disable spill — or add exact-name tool skips — without touching global settings, via its tools_config:

{
  "tools_config": {
    "spill": {
      "enabled": false,
      "skip_tools": ["custom_report"]
    }
  }
}

• Only the literal boolean false disables; malformed or missing settings keep global behavior (fail open)
• skip_tools is additive to the global spill_skip_tools set — agents can add exemptions, never remove global ones
• The executing agent is resolved through code_puppy.agent_execution_context (feat: expose per-agent execution context code_puppy#774); on runtimes without it, the import is guarded and global behavior is unchanged

Tests

Per-agent tests are skipped when the runtime lacks the execution-context module and enabled otherwise:

• against current PyPI runtime: 17 passed, 4 skipped
• against the #774 branch (5d63220d): 21 passed

No CI pin changes needed — core's CI installs the app from main, so the tests un-skip automatically once #774 merges. No production changes outside the spill plugin.