feat(cli): make agentkeys init idempotent

[YoshiyukiSakura]

Change:

• Implements the Stage 8 CLI defensive deliverable for idempotent agentkeys init.
• Adds agentkeys init --force to intentionally re-run initialization when a usable local session already exists.
• Keeps existing library callers compatible through cmd_init, with cmd_init_with_force for forced paths.

Objective Evidence:

• cargo fmt --check (exit code 0)
• cargo test -p agentkeys-cli init_ (exit code 0)
• cargo test -p agentkeys-cli --tests (exit code 0)

Visual Evidence:

• n.a.

Reviewer:

• @repo Intake Reviewer

Risks / Not Covered:

• Visual evidence is n.a. because this is a CLI-only defensive behavior change with no product UI surface or source screenshot.
• This PR intentionally covers only the idempotent init + force CLI slice from issue Stage 8: Production hardening — daemon memory hygiene + CLI defensive features #3.
• Remaining Stage 8 daemon memory-hygiene deliverables are not implemented here.

Refs #3