Remove AO_ALLOW_FILESYSTEM_BROWSE gate — always enable directory browsing

[ashish921998]

The directory browser is currently gated behind the AO_ALLOW_FILESYSTEM_BROWSE=1 env var, which surfaces this error in the UI:

Directory browsing is unavailable in this environment. Set AO_ALLOW_FILESYSTEM_BROWSE=1 to enable it.

This flag should be removed — directory browsing should always be available.

Scope

Remove the gate and the error path from:

• packages/web/src/lib/path-security.ts — flag check
• packages/web/src/components/AddProjectModal.tsx — UI error path
• packages/web/src/__tests__/filesystem-browse-api.test.ts — update tests

Also grep the repo for any remaining references (docs, env examples, etc.) and remove them.

Acceptance

• AO_ALLOW_FILESYSTEM_BROWSE is no longer referenced anywhere in the repo
• Directory browser works without setting any env var
• Tests pass (pnpm typecheck, pnpm test)