chore: install the dependencies with mise install

[viceice]

Changes

• Runs pnpm install from a mise postinstall hook, like the renovate repo does, so mise install sets up the tools and the dependencies in one step. Hooks need mise's experimental setting.
• Adds a development section to the README with the setup and the lint, test and workflow checks.

Context

Please select one of the following:

• This closes an existing Issue, Closes: #
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

AI assistance disclosure

Did you use AI tools to create any part of this pull request?

Please select one option and, if yes, briefly describe how AI was used (e.g., code, tests, docs) and which tool(s) you used.

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• Yes — substantive assistance (AI-generated non‑trivial portions of code, tests, or documentation).
• Yes — other (please describe):

Claude Opus 5.5 (Claude Code) added the hook, wrote the README section and checked that mise install runs pnpm install.

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Documentation (please check one with an [x])

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I have verified these changes via:

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