fix(compilation): raise clear error when managed_section file is missing

[yashpal-2004]

Problem

When agents_md_mode: managed_section is enabled but the target output file (e.g., AGENTS.md) does not exist yet, the compiler falls back to checking for markers in an empty string. This triggers a confusing 'markers not found' error, even though the actual issue is that the file itself is missing.

Proposed Changes

• Added an explicit check for target file existence in _write_output_file_with_config when managed_section mode is enabled.
• If the file does not exist, raise a specific, descriptive ManagedSectionError:
  AGENTS.md does not exist yet. Create it with markers first, or use mode: full for initial generation.
• Added unit test test_write_output_file_managed_section_missing_file to tests/unit/compilation/test_managed_section.py to ensure correct behavior and error message generation.

Verification

• Ran the unit tests locally under a Python 3.12 virtual environment:
  pytest tests/unit/compilation/test_managed_section.py (all tests passed, including the new test).
• Ran ruff check on the modified files (no linting/formatting errors).

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR improves managed-section compilation behavior by failing fast with a clear error when the target output file doesn’t exist, and adds a unit test to validate the error formatting/message.

Changes:

• Raise ManagedSectionError when agents_md_mode="managed_section" and the output file is missing.
• Add a unit test asserting the missing-file managed-section behavior and message formatting.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/unit/compilation/test_managed_section.py	Adds coverage for the new managed-section missing-file error behavior.
src/apm_cli/compilation/agents_compiler.py	Enforces that managed-section updates require an existing output file and raises a clearer error if not.

Comments suppressed due to low confidence (1)

src/apm_cli/compilation/agents_compiler.py:1265

• This wraps the error with [{target}], which will render the full path (e.g., [/tmp/.../AGENTS.md]). The existing test context/comment suggests the bracket prefix should be just the filename (e.g., [AGENTS.md] ...). Consider changing the wrapper to use target.name (or a repo-relative path, if that’s the established convention) so the bracket format stays consistent and stable across environments.

            except ManagedSectionError as exc:
                raise ManagedSectionError(f"[{target}] {exc}") from exc

[yashpal-2004]

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