Add `bt init` command with managed .bt directory layout#98

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Parker Henderson (parkerhendo) wants to merge 8 commits intomainfrom

Commits on Apr 6, 2026

feat(init): Improve structure and management of repo-local .bt with gitignore
Parker Henderson (parkerhendo)
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runtime: store generated artifacts under .bt/cache and .bt/state
Parker Henderson (parkerhendo)
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bt_dir: harden repo layout writes and keep gitignore idempotent
Parker Henderson (parkerhendo)
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runners: unify functions artifacts under shared cache path
Parker Henderson (parkerhendo)
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eval: write runner cache at repo root with guarded materialization
Parker Henderson (parkerhendo)
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setup: avoid writing .bt layout when update is declined
Parker Henderson (parkerhendo)
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refactor: consolidate js and py runner modules into runner
Parker Henderson (parkerhendo)
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refactor(config): extract config lookup and require config file
Parker Henderson (parkerhendo)
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