feat: allow embedding model configuration

[Teagan42]

What

• allow overriding dense and sparse embedding models via env vars or CLI flags
• document model override options
• add tests for CLI and env model configuration

Why

• enables selecting different embedding backends at runtime

Affects

• mcp_plex.loader and mcp_plex.server model initialization and CLI options
• documentation and developer guidelines

Testing

• uv run ruff check .
• uv run pytest

Documentation

• updated README and AGENTS notes

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https://chatgpt.com/codex/tasks/task_e_68bfb7d5f4888328ac273710d36b9b7a

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
mcp_plex
loader.py	196	15	92%	32–34, 306, 309–312, 357–366, 652
server.py	222	12	95%	81, 99–100, 210, 438, 442, 462–466, 513
TOTAL	521	27	95%

Tests	Skipped	Failures	Errors	Time
25	0 💤	0 ❌	0 🔥	30.774s ⏱️