fix(embeddings): respect HERMES_HOME for fastembed cache default

[mxfng]

The MNEMOSYNE_FASTEMBED_CACHE_DIR override (added in fix/ci-embedding-model-cache) lets CI redirect the fastembed model cache, but its default still hardcodes ~/.hermes and ignores HERMES_HOME.

Problem

Users who relocate HERMES_HOME — e.g. to ~/.config/hermes for XDG cleanliness — end up with a stray ~/.hermes/cache/fastembed directory holding ~64MB of BGE model weights, separate from the rest of their Hermes state (config, db, logs, mnemosyne data), all of which correctly live under $HERMES_HOME.

Because the cache path is passed as an explicit cache_dir= argument to TextEmbedding(), no fastembed env var can redirect it — the explicit arg wins. So the default itself has to honor HERMES_HOME.

Fix

Default the cache to <HERMES_HOME>/cache/fastembed, falling back to ~/.hermes/cache/fastembed when HERMES_HOME is unset.

_FASTEMBED_CACHE_DIR = os.environ.get(
    "MNEMOSYNE_FASTEMBED_CACHE_DIR",
    os.path.join(
        os.environ.get("HERMES_HOME", os.path.expanduser("~/.hermes")),
        "cache",
        "fastembed",
    ),
)

This matches the HERMES_HOME handling already used elsewhere in the package (mcp_tools.py), and is a no-op for anyone on the default layout (HERMES_HOME unset → identical ~/.hermes path). The explicit MNEMOSYNE_FASTEMBED_CACHE_DIR override still takes precedence.

Testing

• ast.parse clean.
• Verified locally on a relocated HERMES_HOME=~/.config/hermes: _FASTEMBED_CACHE_DIR resolves to ~/.config/hermes/cache/fastembed, the model loads from there, and embed() returns a (1, 384) vector. No ~/.hermes directory is recreated.

[AxDSan]

Sorry for the delay, was IRL busy the last few days.

Nice catch. The default ~/.hermes/cache/fastembed path was the only place I leaked Hermes state outside $HERMES_HOME for users on a non-default layout. Confirmed the fix matches the HERMES_HOME pattern used in mcp_tools.py, beam.py, banks.py, memory.py, and memory_browser.py, so this is a consistency fix, not a behavior change for the default install.

Verified locally on pr/366:

• python3 -m py_compile mnemosyne/core/embeddings.py clean.
• With HERMES_HOME unset: _FASTEMBED_CACHE_DIR resolves to /root/.hermes/cache/fastembed (same as main).
• With HERMES_HOME=/tmp/hermes-test: resolves to /tmp/hermes-test/cache/fastembed.
• pytest tests/test_optional_embeddings.py tests/test_embedding_optout.py tests/test_embeddings_multilingual.py -> 24 passed in 2.71s.
• CI green: 6/6 jobs SUCCESS (build, docs-check, test on 3.10/3.11/3.12/3.13).

Two small asks before merge:

1. CHANGELOG entry under [Unreleased] for the fix. Even one line ("respect HERMES_HOME for fastembed cache default") keeps the release notes honest.
2. No version bump. That is fine for this, since it is a no-op for default layouts and a correctness fix for non-default ones. Just noting I am not asking for a bump here.

Diff is small, scoped, and the comment block at the top of the change is actually useful. Approving.

[mxfng]

Added the CHANGELOG entry under [Unreleased] (no version bump, per your note). Also rebased onto main to pick up v3.10.1; resolved a CHANGELOG conflict from the new [3.10.1] section. Ready to merge. Thanks for the review!