Correct native dependency count after sqlite-vec addition

[jeremylongshore]

Problem

Upstream v1.2.0 added sqlite-vec beside better-sqlite3 and fs-ext, but live source documentation and diagnostics still describe one or two native dependencies. The downstream mirror review caught the mismatch.

Scope

• Correct current README/plugin manifest/developer guidance/installer and build diagnostics to describe the three external native modules accurately.
• Add an [Unreleased] changelog entry.
• Preserve historical release notes and runtime behavior.
• Do not rebuild the bundle, publish, or change credentials/registries.

Acceptance

A repository-wide scan has no live pre-v1.2 native-count claims outside frozen historical changelog text; formatting and relevant tests pass.

[jeremylongshore]

PR #62 merged as 3da01fd2e0ff3d5bfc9bb23680bbb79ac9e76a6e and corrected the two downstream-mirrored user-facing surfaces (README.md and .claude-plugin/plugin.json) with an [Unreleased] entry. Keep this issue open for the remaining developer-only stale wording in AGENTS.md, bin/init.mjs, build.mjs, and smoke/b1-inbox-sweep.mjs; PR #56 currently overlaps the first two files and should settle before that follow-up. Historical changelog text remains frozen.