fix(sync): guard optional bash profile helper

[andrei-hasna]

Summary

• canonicalize the exact unguarded . "$HOME/.local/bin/env" line while ingesting the managed bash-profile source
• store a readable-file guard so login shells stay silent when the optional helper is absent
• preserve environment initialization when the helper exists

Root cause

The canonical bash-profile config stored an unconditional source of an optional helper. Instructions then faithfully rendered those source bytes into ~/.bash_profile, causing every login shell to emit a missing-file error on machines without the helper.

Regression proof

• before the production change, both absent-helper and present-helper tests failed because the stored source remained unguarded
• after the change, focused suite: 28 pass, 0 fail
• full suite: 623 pass, 0 fail
• typecheck: exit 0
• build: exit 0
• staged and pre-push gitleaks: no leaks found
• package-manager scan: clean

Scope

Only the known bash-profile source and its regression tests change. This PR does not merge, publish, install, apply a profile, or modify the rendered home file.

Task: cde9f87b-ff03-45d2-9882-4899f0ba1e8b

Fixes #65

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