Rewrite cross-note .md links to Jekyll permalinks at build time

[herohua]

Summary

Currently the build script only rewrites image paths. Cross-note links like `taste` are emitted verbatim, so they 404 on the published Jekyll site (which serves posts at `/{stem}` per the site's `/:title` permalink config).

This patch adds a regex pass over each note's body that rewrites `label` (with optional leading `./`) to `label`. Anchors are preserved. Image links are excluded by the leading-`!` lookbehind.

A first pass collects the set of stems whose front matter has `publish: true`. Links to non-publishable or missing targets are left untouched and logged as a WARN to stderr — useful signal without breaking the build.

Test plan

• Local run against current `notes/` (including the new `growing-as-an-engineer-in-the-ai-era.md` from Add note: Growing as an engineer in the AI era #8): both `taste-in-software.md` and `evaluating-a-self-healing-content-agent.md#3-...` rewrote to `/taste-in-software` and `/evaluating-a-self-healing-content-agent#3-...`. No false positives elsewhere in the body.
• Image rewrite still works (the existing `IMAGE_PATH_RE` pass is unchanged and runs first).
• After merge, retroactively fixes the existing `taste-in-software.md` link in Add note: Growing as an engineer in the AI era #8.