Update README for v0.2.x features and new advisory URL

[TooFastTooCurious]

Summary

Three targeted README updates to match current behavior after v0.2.0 and v0.2.1.

Changes

1. Drop the static "current advisories" table. It listed only ABOM-2026-001 and was going to keep drifting as new advisories land. Point readers at the published JSON instead.
2. Update the Auto-updated bullet to reference `advisories.juliet.sh` (the v0.2.1 endpoint) while still linking the contribution repo.
3. Document the auto SHA→tag resolution behavior from fix: SHA-pinned refs at fixed versions should not be flagged as compromised #8. This is the biggest behavior change in v0.2.0 and previously had no documentation. Users seeing the old "verify manually" annotation wouldn't know it's now cleared automatically when the resolved tag is outside the affected range.
4. Reword the "Fast" feature bullet to make it clear that `raw.githubusercontent.com` is about fetching action content (not advisories, which now live at `advisories.juliet.sh`).

Not included (deferred)

• A rendered human-readable advisory index on advisories.juliet.sh itself. Right now we link to the raw JSON, which isn't great UX. Worth a follow-up once abom-advisories gains per-advisory HTML pages.