feat(cli): support APFEL_DEBUG env var for debug logging (#164)

[Arthur-Ficial]

Draft PR - needs @franzenzenhofer review and local test run before merging.

Fixes #164.

Root cause

apfel supports a full family of APFEL_* environment variables (APFEL_PORT, APFEL_TOKEN, APFEL_SYSTEM_PROMPT, etc.) but --debug was only available as a CLI flag. Users who always want debug output - e.g. when integrating apfel into pipelines or tools like MacParakeet - had to pass --debug on every invocation. A APFEL_DEBUG env var was the missing piece.

The fix

Added APFEL_DEBUG environment variable support in CLIArguments.parse(), following the exact same pattern as the other APFEL_* env vars. When APFEL_DEBUG is set to any non-empty value (e.g. 1, true, yes), debug logging is enabled. Empty string is treated as unset. The --debug CLI flag still works and takes precedence (CLI flags are parsed after env defaults, matching existing behaviour).

Also added APFEL_DEBUG to the help text's ENVIRONMENT section.

Test coverage

• Added CLIArgumentsTests: "APFEL_DEBUG env enables debug" - basic APFEL_DEBUG=1 case
• Added CLIArgumentsTests: "APFEL_DEBUG env with any non-empty value enables debug" - APFEL_DEBUG=true
• Added CLIArgumentsTests: "APFEL_DEBUG env empty string does not enable debug" - empty string guard
• Added CLIArgumentsTests: "--debug CLI flag works without APFEL_DEBUG env" - confirms existing flag still works
• Existing --debug sets debug test at line 268 covers the original flag path

What I verified (static only)

• Diff is 3 files, 24 lines added, zero lines removed
• Follows the identical env var pattern used by every other APFEL_* variable in CLIArguments.parse()
• --debug CLI flag (line 306) runs after env defaults (line 191), so CLI always wins - same precedence as --port vs APFEL_PORT
• No new dependencies, no changes to Package.swift
• No touches to .version, BuildInfo.swift, release scripts, CI workflows, or guardrail files
• Swift 6 concurrency: no new shared mutable state - debug is a plain Bool on a Sendable struct, written once during parsing

What I did NOT verify

• Functional correctness. This needs make test on a Mac with Apple Intelligence. I cannot run FoundationModels code. If the fix does not actually resolve the reported behaviour, that is on me to refine - ping me in a review comment.

Anything that seemed suspicious

Nothing - straightforward enhancement request from a legitimate user with a clear use case (MacParakeet dictation cleanup).

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cc @franzenzenhofer - draft stays draft until you review, test locally, and mark ready.

Generated by the apfel bug-solver routine.

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Generated by Claude Code

[Arthur-Ficial]

Superseded by the v1.5.0 release, which implements APFEL_DEBUG (issue #164) on main with tests and docs: https://github.com/Arthur-Ficial/apfel/releases/tag/v1.5.0. Closing this duplicate.