Docs: board pack schema changelog (v1 → v5) for importers

[telivity-otaip]

Why

Board packs are the portable career truth (CareerOps_board_pack.json). Migrators live in code (migrateV1toV2 … migrateV4toV5), but there is no human-readable changelog. Contributors and self-hosters need to know what each bump added before writing importers/plugins.

Size

S — new short doc + one README link.

Task

1. Add docs/BOARD_PACK.md with a table/changelog for schema versions 1 → 5:
   • what fields appeared
   • what never travels (API keys / secrets)
   • pointer to buildBoardPack / importBoardPack in web/lib/board-pack.mjs
2. Link it from docs/README.md and the Export/import section of web/README.md.
3. Keep doctrine explicit: keys never export/import; no invented experience.

Acceptance

• docs/BOARD_PACK.md exists and matches migrators in code (cite functions/line themes, not vibes)
• Current version called out as 5
• Linked from docs index + web README
• No private deploy secrets or internal vault paths

Out of scope

Rewriting the pack format or bumping to v6.

[sethuramansethu008-debug]

Hi! I'd like to work on this issue. I'll document the v1 → v5 board pack schema changes based on the migrators in web/lib/board-pack.mjs and add the requested README links.