Harden error handling + code-review quick wins (A+B)

[devdanzin]

Summary

Works through the still-live items from doc/code-review-findings.md (several originally-listed items were already fixed).

(A) Error-handling hardening

• C2 — Agent.__del__ no longer calls the raising send() on Ctrl-C (the agent is already inactive there, so send() raised AgentError and masked the interrupt); now best-effort and never escapes __del__.
• H1 — python_source.on_session_start catches Exception (not BaseException) around loadModule, so a slow/wedged import stays interruptible.
• H2 — list_all_modules._process_package drops the redundant except (ImportError, Exception); catches Exception and skips the package instead of aborting discovery.
• H3 — utils.remove_logging_pycache is best-effort: guards a missing __pycache__ (was an uncaught startup crash) and wraps the reload.
• H4 + M — Application.exit() runs deinitX11()/config=None in finally; runProject()'s teardown destroy() is guarded so it can't leak X11 access or mask an in-flight exception.
• Policy — exception chaining (raise X from e) at the re-raise sites in config.py, process/create.py, session_directory.py.

(B) Quick wins

• Removed the fragile runtime import cycle: argument_generator imports fusil.python.tricky_weird directly instead of the fusil.python package hub.
• oom_dedup._BT_SKIP now skips the eval-loop teardown / generic-dealloc detector frames, in lockstep with the catalog's gen_known_sites.GENERIC_DETECTOR_FUNCS.
• Retired the broken standalone test_doc.py (stale Py2-era doc/*.rst doctests); the surviving green module doctests (fusil.tools, fusil.process.tools) now run under unittest via tests/python/test_doctests.py.

Test plan

• Full suite: 320 tests OK (+6: 5 doctests wired into unittest, 1 new oom_dedup detector-skip test).
• ruff check + ruff format --check clean over fusil/ tests/ fuzzers/fusil-python-threaded.
• --help smoke confirms the import-cycle change keeps the package init + option parser building cleanly.

Remaining backlog (complexity refactors, softer M-series sentinel warnings, legacy-coupled dead code) stays documented in doc/code-review-findings.md.

Closes #141