Migrate development tooling to Biome and Bun

[tt-a1i]

Summary

• replace Prettier and package-lock with Biome 2.5.8 and Bun 1.3.14
• add a conservative Biome recommended lint gate with explicit legacy-debt exclusions
• keep Node as the runtime and retain node:test, Vitest, TypeScript/Effect checks, and npm package smoke tests
• update CI and contributor documentation for frozen Bun installs
• isolate formatter-only churn in its own commit

Verification

• clean temporary checkout: bun install --frozen-lockfile
• bun run check
• bun run test (591 Node tests, 29 Vitest tests)
• npm pack --ignore-scripts (121 files; no tests, specs, docs/design, tsconfig, Biome/Bun config, or package lock leakage)
• git diff --check

Closes #4

[tt-a1i]

Verification status

• Node 22.19 and Node 24 CI matrices pass on both push and pull_request runs
• frozen Bun install, Biome formatting/lint, TypeScript + Effect checks, 591 Node tests, and 29 Vitest tests pass
• existing npm pack/install and Pi discovery smoke gates remain green
• formatter-only changes are isolated in commit 48c7951; toolchain/CI/docs changes are in fc982e3

Final read-only review is still running; I will follow up if it finds an actionable issue.

[tt-a1i]

Formatter gate clarification

A reviewer suggested adding --check to biome format, but Biome 2.5.8 does not expose that flag (biome format --help lists only --write for mutation). Its default mode is the check mode: before applying the formatter commit, bun run format:check reported 14 files and exited 1; after formatting it exits 0. Therefore "format:check": "biome format ." is the canonical non-mutating CI gate for the pinned version.