Prompt for working directory on prefixed AI CLI start/resume

[Copilot]

Starting or resuming an AI CLI session could inherit default-directory from an unrelated Emacs context, including $HOME. This change makes prefixed start/resume flows ask for the working directory explicitly while preserving the existing args prompt.

• Behavior change

  • C-u on start/resume now keeps the current CLI-args prompt and then prompts for a working directory.
  • The directory prompt is prefilled from the existing project-aware default, so users can accept it or override it.

• Shared startup flow

  • Moved working-directory resolution in the shared backend infra to occur after CLI arg resolution.
  • Applied the behavior centrally so generic CLI backends inherit it without per-backend special handling.

• Resume/session selection

  • Resume picker now prefers the last accessed matching session buffer, so a custom working directory selected during resume continues targeting the intended session.

• Docs and coverage

  • Updated affected start/resume docstrings to describe the new C-u behavior.
  • Added focused tests for prompt ordering, directory prompting, and resume-buffer selection.

;; Before:
;; C-u C-c a a  => prompt for CLI args only

;; After:
;; C-u C-c a a
;;   1. prompt for CLI args
;;   2. prompt for working directory (prefilled with current default)

[Copilot]

Pull request overview

This PR improves native CLI backend session startup/resume reliability by ensuring prefixed start/resume flows explicitly prompt for a working directory (avoiding accidental inheritance of unrelated default-directory), while preserving existing CLI-args prompting behavior.

Changes:

• Updates shared backend infra startup flow so CLI args are resolved first, then (when prefixed) the working directory is prompted with a project-aware default.
• Enhances resume picker behavior to prefer the last accessed matching session buffer for a backend.
• Updates docstrings and adds ERT coverage for prompt ordering, directory prompting, and resume-buffer selection.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
ai-code-backends-infra.el	Adds optional working-directory prompting and reorders startup resolution; prefers last accessed session buffer in resume picker.
ai-code-backends.el	Updates user-facing start/resume docstrings about prefix behavior and adds a compile-time var declaration.
ai-code-opencode.el	Updates resume docstring to reflect new prefix behavior.
ai-code-kilo.el	Updates resume docstring to reflect new prefix behavior.
test/test_ai-code-backends-infra.el	Adds/updates tests for directory prompting, prompt ordering, and resume picker buffer preference.