Address review feedback on actor handling and push semantics

[Copilot]

Summary

This update addresses review feedback on the actor-aware git identity work by tightening actor typing/validation and simplifying the user push path. The goal is to make actor behavior explicit, fail-closed on invalid inputs, and reduce ambiguous push APIs.

• Actor model cleanup

  • Replaced duplicated Literal + constants pattern with a single GitActor enum (USER, AGENT) in git_service.
  • Kept callsites aligned to explicit actor values where actor routing matters.

• Explicit/fail-closed actor checks

  • git_service._actor_env() now explicitly branches on USER/AGENT and raises on unsupported actor values.
  • github_service._token_for_actor() now explicitly handles USER vs AGENT; unsupported actors raise GitHubError.

• Push API narrowing

  • Simplified git_service.push() to user/base-branch semantics only (no remote/branch args).
  • Agent branch pushes continue through push_branch(); human writes/autosave continue through push().

• Operational safety

  • Added once-per-process, thread-safe warning when AGENT GitHub token is missing and fallback token is used (avoids log spam).

• Test updates

  • Updated affected tests/call expectations for the narrowed push() contract.
  • Added/kept focused coverage for actor validation paths.

class GitActor(str, Enum):
    USER = "user"
    AGENT = "agent"

def push(retries: int = 2) -> None:
    # user-only base-branch push helper
    _run("push", settings.git_remote, settings.git_branch, actor=USER_ACTOR)

Context System Checklist

• I updated docs when behavior/semantics changed.
• If API behavior changed, I updated related product/spec docs.
• If reliability/security assumptions changed, I updated runbooks/topical docs.
• I verified generated artifacts are up to date (python3 scripts/generate_context_artifacts.py).
• I ran docs lint checks (python3 scripts/docs_lint.py).

Docs Impact

• No docs impact
• Docs impact present in this PR

If no docs impact, explain why:

This change is a service-layer refactor and guardrail tightening (typing/validation/push helper contract) without changing external API request/response semantics or runbook procedures.

Test Plan

• Unit tests
• Integration/manual test

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[yashturkar]

@yashomdighe copilot addressed your comments from #6