fix(packaging): name the extra when an optional entry point is missing

[r0h1tb]

Description

[project.scripts] installs ast-rag-mcp and ast-rag-watch on every
pip install ast-rag, but mcp and watchdog ship only in the [mcp]
extra. Both commands died at import with a bare ModuleNotFoundError
that never mentions the extra that fixes it.

Each shim now guards on find_spec and falls back to a main that prints
the install line. ast_rag.cli was already fixed this way in the
test_optional_watchdog work; this covers the two entry points that test
does not reach.

On a clean pip install -e . with neither package present:

before:  ModuleNotFoundError: No module named 'mcp'        (traceback, exit 1)
after:   The AST-RAG MCP server needs the optional 'mcp' package, which is not installed.
         Install it with:  pip install 'ast-rag[mcp]'      (exit 1)

Related Issue

None filed — found while auditing the install path.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature
• Breaking change
• Documentation update

Checklist

• My code follows the code style of this project
• I have added tests that prove my fix works
• All new and existing tests passed
• I have updated the documentation accordingly
• I have run ast-rag evaluate --all — needs live Neo4j/Qdrant; unaffected by this change (no query path touched)
• My changes generate no new warnings

Testing

New tests/test_console_scripts_without_extras.py blocks mcp and watchdog
via a meta_path finder in a subprocess, then asserts every [project.scripts]
target still resolves, and that the two optional ones exit naming the extra
rather than raising.

Without the fix, 4 of its 5 tests fail with the reported symptom:

FAILED test_console_script_imports_without_optional_extras[ast-rag-mcp-ast_rag.ast_rag_mcp:main]
FAILED test_console_script_imports_without_optional_extras[ast-rag-watch-ast_rag.watcher:main]
FAILED test_optional_script_names_the_extra_instead_of_crashing[ast-rag-mcp-...]
FAILED test_optional_script_names_the_extra_instead_of_crashing[ast-rag-watch-...]

Suite: 255 passed / 1 skipped / 1 xfailed before, 260 / 1 / 1 after (5 new).
Also verified through a real uv pip install -e . into an empty venv, not
only the subprocess simulation — that produced the before/after above.