release: v0.4.0 — Exa search adapter

[askalf]

Summary

Cuts v0.4.0 to ship the Exa search adapter from #19 (community contribution by @tgonzalezc5). Also validates the inline-publish chain documented in v0.3.0's pending CHANGELOG entry — this is the first deepdive release that goes end-to-end through `auto-release.yml` without manual maintainer intervention.

Changes

• `package.json`: 0.3.0 → 0.4.0
• `CHANGELOG.md`: convert pending [Unreleased] section to [0.4.0] heading; add Added / Exa adapter entry crediting @tgonzalezc5

Test plan

• Local build green (typecheck + tsc)
• Local test suite green (212/212 from PR feat: add Exa AI-powered search tool #19's verification, including 14 new Exa adapter tests)
• PR feat: add Exa AI-powered search tool #19 merged into master 26 minutes ago — CHANGELOG mirrors the merge state
• Watch `auto-release.yml` chain on merge: PR merge → build + typecheck + test + `--help` smoke → `gh release create` v0.4.0 → `npm publish --access public --provenance`
• Confirm v0.4.0 lands at `@askalf/deepdive` on npm with SLSA provenance attestation