chore: prepare release 0.8.4

[titouanmathis]

Prepare the 0.8.4 patch release.

• Bump version 0.8.3 → 0.8.4 (no breaking changes → patch, per the 0.x policy).
• Stamp CHANGELOG.md: move the [Unreleased] entries under ## [0.8.4] and repoint the compare links.

Rolls up:

• On-disk retrieval of size-dropped diffs on by default + skip-callout dedupe ([feat: default on-disk retrieval of skipped diffs; dedupe skip callout #129]).
• Moving MAJOR / MAJOR.MINOR action-pinning tags ([feat: maintain a moving minor tag for action pinning #127], [feat: also maintain a moving major tag for action pinning #128]).

Merging this, then pushing the 0.8.4 tag, triggers the npm publish + GitHub release via publish.yml.

[github-actions]

Code Review

Risk: Low — This is a straightforward release-prep change updating the package version and changelog metadata; the visible diff is safe to merge.

This MR prepares the 0.8.4 patch release by bumping package.json from 0.8.3 to 0.8.4, adding the 0.8.4 changelog section, and repointing the compare links at the bottom of CHANGELOG.md. The visible changes match the stated release intent, and I did not find any blocking defects in the reviewed files.

Notes:

• package-lock.json was skipped by the diff size limit and was not reviewed.

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Review usage: 6,079 in (3,584 cached) / 388 out tokens — $0.0130 (cloudflare-ai-gateway/gpt-5.4, thinking: medium)

Reviewed by @weareikko/code-review v0.8.4 for commit 3ef475b.