docs(docs): add strategy, coding, and system design documentation hierarchy

docs/content/02-guides/01-coding/09-decision-space/index.mdx

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+---
+title: 'Decision-Space'
+description: 'Nodes are states, not data. The structure does not exist until you walk it.'
+sidebar:
+  order: 1
+  label: Introduction
+---
+
+:::warning[Under construction]
+This page is a scaffold. Notes, canonical problems, and template code are not written yet.
+:::
+
+## Patterns
+
+- [Backtracking](/guides/coding/decision-space/backtracking) — DFS over the state tree with undo; exhaustive enumeration.
+- [Dynamic Programming](/guides/coding/decision-space/dynamic-programming) — Backtracking plus memoization over overlapping subproblems.
+- [Greedy](/guides/coding/decision-space/greedy) — DP where the local optimum provably suffices, so no memo is needed.
+
+**Systems anchor:** DP memo is an idempotency cache; backtracking is a saga with compensating undo; greedy is local decisions that converge.