Refactor pc configure for project stores, synchronization, and reconciliation

Follow-up to #100 and #107. The backend configuration/storage contracts landed in 8f07a26 and 08ff39c; this issue is intentionally limited to the full interactive pc configure UX refactor.

Scope

• Present the unified ~/.pc/config.json model without legacy-home migration.
• Configure project-store sync enablement, remote, branch, poll interval, jitter, and retry/backoff.
• Configure the trusted reconciliation runner as typed program/argv plus explicit prompt transport, timeout, log cap, and retention.
• Validate runner configuration without shell-string interpolation.
• Make defaults, current values, disabled states, and validation errors easy to inspect before saving.
• Keep hook preflight side-effect-free; this remains an interactive command only.
• Update help and repository-management guidance for the final interaction.

Design questions

• Whether sync/reconciliation settings remain global defaults only or support explicit per-project overrides.
• Whether known harnesses get convenience adapters in addition to the generic typed argv editor.
• Whether remote bootstrap/binding belongs inside pc configure or stays under pc project attach/sync.

Acceptance criteria

• Every project-store sync/reconciliation setting can be inspected and edited safely.
• Reconciliation commands are stored as typed argv with explicit prompt transport.
• Invalid or incomplete runner configurations are rejected before writing config.
• Existing config is preserved on cancellation or validation failure.
• Help/docs and tests cover fresh configuration and reconfiguration.
• No compatibility or migration path for ~/.proactive-context is introduced.

Start at the interactive pc configure entry point and inspect the unified ~/.pc/config.json model and the backend contracts from commits 8f07a26 and 08ff39c. Resolve the listed design questions before implementation, then review the existing help, repository-management guidance, and tests. Done means all sync and reconciliation settings are safely editable, invalid configurations are rejected before writes, cancellation preserves existing config, and help and tests cover fresh configuration and reconfiguration.