Add contributing guide

[kitsuyui]

Summary

• Add a CONTRIBUTING guide with setup, formatting, lint, type-check, test, and build commands.
• Document basic expectations for focused pull requests and local verification.

Verification

• bun run format
• git diff --check
• bun run lint
• bun run typecheck
• bun run test

Related to kitsuyui/kimono#104.

[github-actions]

Code Metrics Report

	main (15fc06b)	#1594 (2e0a4d7)	+/-
Coverage	87.2%	87.2%	0.0%
Code to Test Ratio	1:0.5	1:0.5	0.0
Test Execution Time	6s	6s	0s

Details

  |                     | main (15fc06b) | #1594 (2e0a4d7) | +/-  |
  |---------------------|----------------|-----------------|------|
  | Coverage            |          87.2% |           87.2% | 0.0% |
  |   Files             |             11 |              11 |    0 |
  |   Lines             |            141 |             141 |    0 |
  |   Covered           |            123 |             123 |    0 |
  | Code to Test Ratio  |          1:0.5 |           1:0.5 |  0.0 |
  |   Code              |           1119 |            1119 |    0 |
  |   Test              |            614 |             614 |    0 |
  | Test Execution Time |             6s |              6s |   0s |

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Reported by octocov