Extract tryReadFile to a shared utility in build-cli

Summary

build-tools/packages/build-cli/src/commands/ai.ts defines a tryReadFile helper (lines 344-360) that reads a file and returns undefined on ENOENT, rethrowing on all other errors. The same ENOENT-swallow pattern already exists in build-tools/packages/build-cli/src/library/pnpmCatalog.ts (lines 27-33).

Proposal

Extract tryReadFile into a shared location — src/library/text.ts already houses file I/O helpers (readLines, writeFileWithLineFeeds) and is a natural home. Then update both ai.ts and pnpmCatalog.ts to import from the shared location.

Context

Identified during code reuse review of the flub ai PR. Deferred because the refactor touches code outside the PR's scope.