fix(detect): forward follow_symlinks from detect_incremental to detect

[alphanury]

Problem

detect_incremental(root) always called detect(root) without forwarding the follow_symlinks kwarg added in #33. As a result, corpora that include symlinked sub-trees pointing to directories outside the scan root were visible to a full detect() run with follow_symlinks=True but invisible to any subsequent --update run.

Concrete failure case

A user has a knowledge corpus laid out like:

```
~/.hermes/graphify-jarvis/
├── state_of_truth -> ~/.hermes/state_of_truth (symlink)
├── memories -> ~/.claude/.../memory (symlink)
└── graphify-out/
```

Initial /graphify <path> build (with the appropriate follow_symlinks=True plumbed in) correctly indexed ~95 files via the symlinks. Subsequent /graphify <path> --update runs only saw 3 files (the contents of graphify-out/ itself) because detect_incremental re-called detect(root) without the flag, silently dropping the symlinked sub-trees. New SoT reports and memories were never picked up by incremental scans.

Fix

Add a keyword-only follow_symlinks parameter to detect_incremental() and forward it to detect(). Default stays False for backwards compatibility — only callers that already opt in to symlink following on detect() pick up the new behaviour for incremental runs too.

Test

Added test_detect_incremental_propagates_follow_symlinks covering the regression: a corpus with a symlinked directory is invisible with follow_symlinks=False, fully indexed with follow_symlinks=True, and correctly reports zero new files on a second incremental scan after the manifest is saved.

```
$ pytest tests/test_detect.py -v
============================== 30 passed in 0.07s ==============================
```

Caller note

The CLI / skill currently doesn't expose --follow-symlinks as a flag, but watch.py already plumbs it through to the full-detect() rebuild path. This PR keeps the same opt-in shape: callers that already pass follow_symlinks=True to detect() will now see consistent behaviour on --update. Wiring a --follow-symlinks CLI flag through to both detect_incremental and the existing detect() callsites would be a small follow-up if you want.