docs: add thinking walkthrough for problems 1-99

[yanglbme]

Summary

• Add a 思考 / Thinking section to every method in problems 1-99, inserted after the method heading and before the existing algorithm write-up.
• Each section walks from the naive idea and its bottleneck to the observation that leads to this method, without restating the code that follows.
• Record the format in AGENTS.md and solution/template.md so later 100-problem batches stay consistent. This is the first of the per-100-problem PRs (next: 100-199).

Test plan

• Spot-check Two Sum, 3Sum, Trapping Rain Water, Climbing Stairs, and Largest Rectangle: thinking matches the actual Solution.py and does not change code tabs.
• Confirm every ### 方法 / ### Solution heading in solution/0000-0099 has a matching 思考 / Thinking block.
• Confirm <!-- solution:start --> / <!-- tabs:start --> markers and all Solution.* files are unchanged.

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