fix: revert auto-assign agents-access role enabled

[johnstcn]

This reverts commit d4a9c63 (#23968).

[coder-tasks]

Documentation Check

Updates Needed

• docs/ai-coder/agents/getting-started.md - Step 3 section contains a duplicate sentence: "Owners always have full access and do not need the role." appears twice. The paragraph currently reads:

  Owners always have full access and do not need the role.
  Repeat for each user who needs access. Owners always have full access and do not need the role.

  The second instance appears to be a leftover from restructuring. Suggest removing the duplicate so the section reads cleanly.

---

Automated review via Coder Tasks

[Copilot]

Pull request overview

This PR reverts the previously-added behavior that auto-assigned the agents-access role to newly-created users when the agents experiment is enabled, restoring the requirement that an owner explicitly grants the role.

Changes:

• Removed agents-access auto-assignment logic from CreateUser().
• Deleted/updated tests that depended on the auto-assignment behavior.
• Updated Agents documentation to reflect manual role assignment for users.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
coderd/users.go	Removes the auto-assign agents-access role block from user creation.
coderd/users_test.go	Drops the test that asserted auto-assignment under the agents experiment.
coderd/exp_chats_test.go	Simplifies “no agents-access” cases now that new users won’t be auto-granted the role.
enterprise/coderd/userauth_test.go	Removes enabling the agents experiment in an OIDC role-sync test (no longer needed).
docs/ai-coder/agents/getting-started.md	Updates guidance to require explicitly granting the “Coder Agents User” role.
docs/ai-coder/agents/early-access.md	Updates early access steps to reflect manual role granting.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[johnstcn]

Cherry-pick for 2.33 here: #24173