feat(nakama): add cultivation progression

[JOY (JOY)]

Summary

• Add secondspawn_cultivation_event for prototype Nibirium absorption XP.
• Promote bodies from Awakening to Enhancement at 1000 prototype XP, with per-event XP caps and idempotent retry handling through activity IDs.
• Reject cultivation progress for dead bodies until reincarnation creates a fresh body.
• Log accepted cultivation events into bounded agent_activity and expose a Unity Nakama cultivation wrapper.
• Update the pre-alpha changelog.

Verification

• npm run build in backend/nakama
• npm test in backend/nakama
• go test -count=1 ./... in backend/gateway
• go vet ./... in backend/gateway
• npx --yes markdownlint-cli2@0.18.1 "*.md" "docs/**/*.md" ".github/**/*.md"
• git diff --check
• Changed-file em-dash/en-dash scan

Local Review

Local code-review skill pass result: approved with suggestions. No engine specialist is configured in .Codex/docs/technical-preferences.md, so specialist routing was skipped. The change keeps cultivation progression in Nakama game-backend state and does not let LLM output mutate progression.

Caveat

Unity clean-worktree batchmode compile is still blocked by the existing Package Manager path undefined issue in this repo family, so Editor/MCP compile verification remains required before a no-caveat Unity merge claim.

Gemini (@gemini-code-assist) review
Codex (@codex) review

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[JOY (JOY)]

Closing by product decision: cultivation/Nibirium progression is deferred and should not land in the current vertical slice. MVP progression will use level/stats while the deeper body/soul progression mechanic is redesigned later.