Add optional Pup Lint agent tool

[kvandre12-commits]

Summary

• add an optional pup_lint builtin plugin discovered through the existing entry-point system
• register and advertise a structured, diagnostic-only Pup Lint tool when the CLI is on PATH or the module is importable
• add model-facing guidance to lint changed Python paths before completing coding tasks
• execute with argv directly, enforce a timeout, clean up cancelled child processes, and cap returned diagnostics
• remain completely silent when Pup Lint is unavailable

Pup Lint is the zero-Rust, dependency-free Python linter released at:
https://github.com/kvandre12-commits/pup-lint

The model tool intentionally does not expose --fix; model-initiated mutations should continue through Code Puppy's governed file tools.

Verification

• ruff check --fix code_puppy_core_plugins/pup_lint tests/test_pup_lint_plugin.py tests/test_pup_lint_plugin_timeout.py
• ruff format code_puppy_core_plugins/pup_lint tests/test_pup_lint_plugin.py tests/test_pup_lint_plugin_timeout.py
• 10 focused tests pass on Android/Termux
• real cross-repository smoke test discovers Pup Lint, injects the tool, invokes the actual CLI, and returns F401/W291/W292 diagnostics
• built wheel contains the pup_lint entry point, implementation, and README
• full suite: 1,987 passed, 2 skipped; 11 environment-only failures from unavailable Azure Identity and isolated wheel subprocesses that intentionally omit local code_puppy/httpx

[kvandre12-commits]

Revalidated the current PR head (1e9126e) against current Code Puppy main after the subprocess-hardening update.

Validation on Python 3.14:

• 10 passed across tests/test_pup_lint_plugin.py and tests/test_pup_lint_plugin_timeout.py
• Ruff E4,E7,E9,F,I checks passed for the plugin and focused tests
• Ruff formatting check passed
• git diff --check passed

The focused run explicitly supplied the current Code Puppy checkout as the plugin host; the core-plugin repository intentionally does not declare the host application as a runtime dependency. The PR remains mergeable and current with the plugin repository's main.