Standardize Unicode encodings to explicitly use UTF-8

[dvmukul]

Summary of Changes

This Pull Request standardizes file operations across the codebase by adding explicit encoding='utf-8' to open() calls.

Rationale

On Windows, the default system encoding is often locale-specific (e.g., CP1252 or CP950), which causes UnicodeDecodeError when reading or writing files that contain UTF-8 characters (like prompt templates or localized marketplace metadata). This fix ensures that apm behaves consistently across Windows, macOS, and Linux.

Key Fixes:

• script_runner.py: Fixed issue in prompt compilation and script reading (resolves bug: UnicodeDecodeError reading .prompt.md on Windows CP950 — open() missing encoding parameter #604).
• config.py: Standardized global configuration file operations.
• marketplace/client.py & registry.py: Fixed local caching of marketplace manifests.
• models/plugin.py: Ensured stable reading of plugin.json metadata.
• runtime/copilot_runtime.py: Fixed MCP configuration retrieval.

Verification

Changes were audited for text-based text vs binary-based operations to avoid data corruption. Existing unit tests were prioritized to ensure no regressions in core functionality.

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[sergio-sisternes-epam]

Looks good overall — the changes are correct and directly fix the Windows UnicodeDecodeError from #604. Two suggestions in the inline comments (missed open() calls + test coverage).

[sergio-sisternes-epam]

👍 Good fix. Note that an AST scan of the codebase found five more text-mode open() calls that still lack explicit encoding='utf-8':

File	Line	Operation
adapters/client/codex.py	65	toml.dump() — write config
adapters/client/codex.py	80	toml.load() — read config
adapters/client/copilot.py	68	json.dump() — write config
adapters/client/copilot.py	83	json.load() — read config
deps/github_downloader.py	184	write empty gitconfig

Since the PR title says "Standardize … across the codebase," it would be great to cover these too (same pattern, same Windows risk).

[sergio-sisternes-epam]

This is the core fix for #604. Consider adding a small test that round-trips a non-ASCII string (e.g. CJK characters) through PromptCompiler.compile() to lock in the fix — the existing test_cli_encoding.py only covers console stream reconfiguration, not file I/O.