v2.60.0

Add provider/model-specific system prompt overrides

Configure persistent instructions for one exact provider/model pair, either replacing dreb's built-in prompt or appending to it, with the same behavior for built-in and custom/local models.

What changed

• Exact model targeting. Canonical provider/model entries under modelSettings prevent prompt instructions from leaking between providers that expose the same model ID.
• Replace or append. systemPrompt replaces the built-in base when no explicit session replacement is active; appendSystemPrompt composes after existing append sources.
• Complete model lifecycle support. Overrides apply on startup, model selection and cycling, session restore, and /reload, with stale instructions removed when the active model changes.
• Loud validation and atomic switching. Empty, malformed, or contradictory prompt settings fail explicitly before model-switch, restore, or selector state is committed.
• Backward compatibility and documentation. Existing bare-ID thinking-display settings continue to work, and configuration examples cover built-in, custom, and slash-containing model IDs.
• More reliable mach6 reviews. Every review round now considers the full PR, later deltas are supplemental, and failed specialists are retried rather than omitted.

Implemented in PR 483.