chore: defer cultivation progression

[JOY (JOY)]

Summary

• Closes the old cultivation / Nibirium XP direction as deferred design, not vertical-slice scope.
• Removes cultivation/progress_xp/CultivationTier from Gateway, Nakama, and Unity prototype contracts.
• Updates roadmap, GDD, systems index, and agent context docs so level/stats are the current progression baseline.
• Keeps a short deferred design note for future advanced body/soul progression brainstorm.

Review

Local code-review fallback because Gemini/Codex PR review bots are quota-limited on the active PR stack:

• Runtime grep is clean for cultivation, Nibirium, progress_xp, and CultivationTier under backend/ and Unity/Assets/_SecondSpawn/Scripts.
• Hard Rule feat(backend): add Nakama OSS local base #2 remains intact: LLMs still cannot grant authoritative state, and docs now explicitly include level/stat progress in the forbidden direct-grant list.
• Hard Rule feat(nakama): persist agent runtime activity #8 checked: AGENTS.md and .claude/CLAUDE.md are synced except for the sister-file header.

Verification

• npm run build && npm test in backend/nakama
• go test -count=1 ./... && go vet ./... in backend/gateway
• markdownlint-cli2 "*.md" "docs/**/*.md" ".github/**/*.md"
• git diff --check
• rg scan for em dash / en dash

[gemini-code-assist]

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