feat: support {{ env.X }} interpolation in config fields

[felipecrs]

Changes

Important

This PR is WIP. Including this description.

Fixes {{ env.X }} template interpolation in config fields like hostRules.username. Previously, only {{ secrets.X }} and {{ variables.X }} were supported in config interpolation, but {{ env.X }} (which accesses customEnvVariables) was not resolved and would remain as a literal string.

Now applySecretsAndVariablesToConfig also resolves {{ env.X }} patterns using the merged environment (filtered process.env + customEnvVariables + user-configured env).

Context

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• This closes an existing Issue, Closes: #
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

AI assistance disclosure

Did you use AI tools to create any part of this pull request?

Please select one option and, if yes, briefly describe how AI was used (e.g., code, tests, docs) and which tool(s) you used.

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• Yes — substantive assistance (AI-generated non‑trivial portions of code, tests, or documentation).
• Yes — other (please describe):

Claude Opus 4.6 was used to investigate the bug, implement the fix, and write the tests.

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The public repository: N/A