Docs: add real CHANGELOG.md and CONTRIBUTING.md

[egorfedorov]

Problem

There is no real CHANGELOG.md (only a placeholder CHANGELOG-notes.md containing "entry 1..9") and no CONTRIBUTING.md. For a Claude Code marketplace plugin at v4.1.0, release history and contribution guidance matter for trust and adoption.

Proposal

• Add a proper CHANGELOG.md (Keep a Changelog format), backfilling notable v4.0 → v4.1 changes (silent hooks, honest NET savings, big-file map-then-load).
• Add CONTRIBUTING.md: dev setup, npm test, hook architecture overview, how skills map to src/*.js, PR expectations.
• Wire scripts/sync-version.js / prepublishOnly to remind about changelog updates.
• Remove or repurpose the CHANGELOG-notes.md placeholder.

Acceptance

• CHANGELOG.md and CONTRIBUTING.md exist and are accurate for v4.1.

[egorfedorov]

Half done in v4.2.0: a real CHANGELOG.md now exists (4.0.0 → 4.2.0). CONTRIBUTING.md still to be written — leaving this open for that.