reload: pick up the new code, and drop MCP tools that are gone ...

reload_tools reported success while three separate things kept it from
doing what it says.

The bytecode cache is keyed on (mtime in whole seconds, file size), so a
tool edited to the same length inside the same second as its previous
load re-ran the OLD code — the reload was real, the new version was not
live. The loader now compiles the source itself instead of consulting
__pycache__ (importlib.invalidate_caches() does not help here).

MCP registrations only ever grew: register_mcp_tools called
register_external, and unregister_external was used in one place, so a
server removed from the config or a tool a server stopped exposing
stayed in the registry and failed only when the model called it. Reload
now clears external tools and rebuilds them.

enabled.json naming a tool that failed to load (the gmail/html2text case)
was visible only as a log line. It is now part of the report.

The reload itself moves to navi/core/reload.py, one implementation shared
by the tool and the admin route, so the two cannot leave different
toolsets behind. list_tools.py also read enabled.json through its own
cwd-relative path, which made it disagree with the real toolset, and the
class-based loader rejected execute(self, params, ctx=None) — the shape
every built-in uses.

Fix user tool execute() signature to accept ctx keyword argument ...

The tool_executor always passes ctx=ctx when calling tool.execute(),
but _try_module_level in loader.py created user tools with
_execute(self, params) only. This caused:

  Error: _execute() got an unexpected keyword argument 'ctx'

when calling get_current_datetime and any other module-level user tool.

- Add ctx=None parameter to the generated _execute wrapper
- Preserves backward compatibility with existing user tools

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Refactor: move tool helper modules into _internal subpackage ...

Moves non-tool infrastructure out of navi/tools/ root so that only
actual tool classes live there:

  base.py           → _internal/base.py
  loader.py         → _internal/loader.py
  middleware.py     → _internal/middleware.py
  logging_middleware.py → _internal/logging_middleware.py
  _time_parser.py   → _internal/time_parser.py

All imports updated across core/, api/, mcp/, tools/, and tests/.
No proxy files remain in navi/tools/ root.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>