fix(shellenv): preserve newlines in exported env values (ambiguous redirect)

[mikeland73]

Summary

Fixes #2814.

eval "$(devbox global shellenv)" printed bash: 1__bp_interactive_mode: ambiguous redirect at every prompt when the user's environment carried a multi-line PROMPT_COMMAND (e.g. the bash-preexec value some terminals set).

Root cause: exportify in internal/devbox/envvars.go escaped literal newlines in a value as \ followed by a newline. Inside a double-quoted shell string that sequence is a line continuation, which the shell deletes — joining the value's adjacent lines. A line ending in 2>&1 collapsed into the following command __bp_interactive_mode, producing 2>&1__bp_interactive_mode, which bash parses as a redirect to the file 1__bp_interactive_mode → "ambiguous redirect".

A bare newline inside double quotes is already preserved verbatim, so the escape was both unnecessary and the source of the corruption. The fix drops the backslash escape for \n; the other special characters ($, `, ", \) are still escaped.

# before (buggy):  export PC="... 2>&1\<newline>__bp_interactive_mode";  -> value: "... 2>&1__bp_interactive_mode"
# after  (fixed):  export PC="... 2>&1<newline>__bp_interactive_mode";   -> value preserves the newline

How was it tested?

• Added TestExportifyPreservesNewlines asserting the emitted export contains a bare (not backslash-escaped) newline and preserves the multi-line value. Existing exportify tests still pass.
• Reproduced the shell behavior directly: sourcing the previous output collapsed the two lines (... 2>&1__bp_interactive_mode); sourcing the fixed output keeps them as separate commands.
• go build / go vet on internal/devbox are clean.

Community Contribution License

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maintainers under the terms of the
Apache 2 License.

By creating this pull request, I represent that I have the right to license the
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Community Contribution License.

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cc @proedie (issue reporter)

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https://claude.ai/code/session_01XMi8bm4fhxGAdxSCt5YJY3

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[Copilot]

Pull request overview

Fixes an issue in devbox global shellenv where multi-line environment variable values (notably PROMPT_COMMAND) were being corrupted due to incorrectly escaping literal newlines, which could trigger bash: ... ambiguous redirect errors when evaluated.

Changes:

• Stop escaping \n in exportify so literal newlines inside double quotes are preserved verbatim (avoiding POSIX line-continuation semantics).
• Add a regression test asserting the export output contains bare newlines and does not emit backslash-newline sequences.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
internal/devbox/envvars.go	Adjusts exportify escaping rules to preserve literal newlines in exported env values.
internal/devbox/envvars_test.go	Adds a regression test ensuring multi-line values are emitted without \\\n escaping and remain multi-line.

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