chore: add lefthook to run CI checks locally

[kitsuyui]

Summary

• Add lefthook.yml to mirror the CI lint and test checks as local Git hooks
• Add a ## Development section to README.md documenting how to install and use the hooks
• CI workflow files are not modified

What the hooks do

• pre-commit: runs bun run lint (Biome static checks) for fast feedback before each commit
• pre-push: runs bun run lint and bun run test so the full local suite passes before pushing

CI still runs the complete suite on every pull request and push to main; the hooks surface failures earlier in the development workflow.

Files changed

• lefthook.yml (new)
• README.md (Development section added before License)

[github-actions]

🎉 Happy commit!

• dogfooding: PR chore: add lefthook to run CI checks locally #844 always celebrates.
• dogfooding: 9547b0c9d10d6fe7d22a57ed0fe2f9f8fe51b200 always celebrates.

[github-actions]

Code Metrics Report

	main (d6def62)	#844 (50a98ea)	+/-
Coverage	99.2%	99.2%	0.0%
Code to Test Ratio	1:0.5	1:0.5	0.0
Test Execution Time	2s	2s	0s

Details

  |                     | main (d6def62) | #844 (50a98ea) | +/-  |
  |---------------------|----------------|----------------|------|
  | Coverage            |          99.2% |          99.2% | 0.0% |
  |   Files             |              5 |              5 |    0 |
  |   Lines             |            134 |            134 |    0 |
  |   Covered           |            133 |            133 |    0 |
  | Code to Test Ratio  |          1:0.5 |          1:0.5 |  0.0 |
  |   Code              |           1678 |           1678 |    0 |
  |   Test              |            955 |            955 |    0 |
  | Test Execution Time |             2s |             2s |   0s |

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Reported by octocov