Implement plan phase

Execute exactly one phase of a written plan. $ARGUMENTS gives the plan file path and the phase number. The phase boundary is the finish line: the run ends with this phase implemented, verified, recorded in the plan files, and committed.

1. Load the phase

Read the plan file and locate the given phase and its task list. While reading, also note:

• the plan's Status line or progress markers,
• any verification steps the plan specifies for this phase,
• related plan docs the plan references (an index or overview file that tracks this plan's status).

If the plan file or the phase number can't be found, stop and ask.

2. Branch

Run git branch --show-current. If the current branch is already this plan's working branch, continue. Otherwise ask the user whether to create a new branch for this plan — suggest a name following the repo's branch-naming conventions — or stay on the current branch. Wait for the answer before editing anything.

3. Implement