perf(cli): avoid eager API imports

[WilliamK112]

Summary

• import the Obsidian wiki-footer helper directly from its core definition instead of through palinode.api.server
• move _MEMORY_CATEGORY_DIRS beside _TYPE_TO_CATEGORY in palinode.core.memory_write, while preserving the historical API re-export
• add a subprocess regression proving a plain palinode.cli import loads neither FastAPI nor palinode.api.server

Fixes #143.

Import-time measurement

Measured with Python 3.11 using five fresh post-warmup subprocesses per revision:

PYTHONPATH=. python -X importtime -c "import palinode.cli"

• before: median cumulative palinode.cli import time 331 ms (323-367 ms)
• after: median cumulative palinode.cli import time 188 ms (187-210 ms)
• change: 143 ms / 43% lower median cumulative import time

Before this change, the trace included palinode.api.server (about 78-81 ms cumulative in the measured runs). After the change, that module is absent; a direct module check also reports both fastapi and palinode.api.server as unloaded.

Validation

• focused import/Obsidian/frontmatter/wiki-footer tests: 99 passed
• complete suite: 3130 passed, 10 skipped, 5 xfailed
• ruff check palinode/ tests/ scripts/
• bandit -r palinode/ -ll
• git diff --check

AI assistance disclosure: I used OpenAI Codex to help inspect the import graph, implement the scoped change, run measurements, and validate the patch. I reviewed the final diff and evidence before submitting.