fix: fall back to PATH when codex binary missing

[chindris-mihai-alexandru]

Summary

• fall back to PATH lookup when a configured Codex binary path is missing
• keep existing behavior when the configured path works

Motivation

Some installs (Homebrew/npm) surface stale or invalid binary paths. Today that hard-fails sessions even when codex is available on PATH. This change attempts the configured path first, then falls back to PATH for missing binaries.

Testing

• not run (local change)

Note

Fall back to PATH codex binary when configured binary path is not executable

• Adds resolveCodexBinaryPath in codexAppServerManager.ts to validate the configured binary via assertSupportedCodexCliVersion before use.
• If the configured path fails with "is not installed or not executable" and differs from the default "codex", the function retries with "codex" from PATH and logs a console warning.
• startSession now delegates binary resolution to this helper instead of asserting and selecting the path inline.
• Behavioral Change: sessions that previously failed when a non-default binary path was misconfigured will now silently fall back to codex on PATH if available.

^{Macroscope summarized 102c216.}

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