feat: add loader queue sentinels

[Teagan42]

What

• add shared ingestion sentinel and queue type aliases in the loader pipeline helpers
• update LoaderPipeline to rely on the shared queue aliases and accept the new sentinel alongside existing None tokens
• document the sentinel usage for upcoming integration coverage

Why

• centralizing the queue types and sentinel improves consistency between pipeline stages and prepares later stages for sentinel-based coordination without breaking current behavior

Affects

• loader pipeline queue typing and completion signaling

Testing

• uv run pytest tests/test_loader_unit.py
• uv run ruff check mcp_plex/loader/pipeline/channels.py mcp_plex/loader/__init__.py

Documentation

• updated inline module documentation for the loader pipeline queues

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https://chatgpt.com/codex/tasks/task_e_68e2a7a171708328a5b3242413d39d70

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
mcp_plex/loader
__init__.py	816	131	84%	192, 247, 249–251, 267, 353–358, 960–961, 1005, 1007, 1060–1061, 1072–1074, 1077, 1080–1081, 1083, 1105, 1115, 1126–1158, 1173, 1175, 1182–1196, 1199–1213, 1219, 1234–1265, 1270–1303, 1306–1310, 1315–1323, 1333, 1342, 1346–1348, 1467–1469, 1802
mcp_plex/loader/pipeline
channels.py	70	2	97%	19–20
mcp_plex/server
__init__.py	614	29	95%	43–44, 119–120, 148, 252, 256, 277–280, 297, 362, 365, 402, 420–421, 458, 1109, 1131–1137, 1173, 1191, 1196, 1214, 1338, 1375
__main__.py	4	4	0%	3–8
config.py	48	7	85%	50, 52–55, 65, 76
TOTAL	1762	173	90%

Tests	Skipped	Failures	Errors	Time
101	0 💤	0 ❌	0 🔥	56.296s ⏱️