chore(compile): remove legacy `.chatmode.md` primitive type

[sergio-sisternes-epam]

Context

Follow-up to the dogfooding PR (closes #695 / #792). The repo's own agent primitives have been migrated to .apm/agents/*.agent.md and the legacy .chatmode.md format is no longer used anywhere in microsoft/apm.

Primitive discovery still scans for the legacy format everywhere:

# src/apm_cli/primitives/discovery.py
LOCAL_PRIMITIVE_PATTERNS = {
    'chatmode': [
        "**/.apm/agents/*.agent.md",
        "**/.github/agents/*.agent.md",
        "**/*.agent.md",
        # Legacy support (.chatmode.md)
        "**/.apm/chatmodes/*.chatmode.md",
        "**/.github/chatmodes/*.chatmode.md",
        "**/*.chatmode.md",
    ],
    ...
}

Matching legacy patterns also exist in DEPENDENCY_PRIMITIVE_PATTERNS and DEPENDENCY_GITHUB_PRIMITIVE_PATTERNS, plus any related parsing / integrator / test fixtures.

Proposal

1. Deprecation notice -- one release cycle. Emit a [!] warning when any .chatmode.md file is discovered, pointing contributors at .agent.md.
2. Ecosystem sweep -- audit the public marketplace / known APM packages for any consumer still shipping .chatmode.md. Publish a migration guide if found.
3. Removal PR -- drop the legacy patterns from discovery.py, remove the chatmode integrator fallback, and update tests / docs.

Removing the format simplifies discovery, shrinks the test surface, and removes the chatmode / agent naming confusion for new contributors.

Open questions

• Do we want the deprecation warning gated on target=all / target=copilot specifically, or unconditional?
• Is there data on how many published packages still use .chatmode.md? If zero, we can skip straight to removal.

Labels

breaking-change, compilation, tech-debt