fix: stale model pricing, context sizes, and missing models in handle_grok_models

[hrco]

Problem

The model info hardcoded in handle_grok_models() (server.py:1213-1267) no longer matches the xAI docs.

Pricing/Context discrepancies

Model	xBridge says	xAI docs say
grok-4.20-0309-reasoning	2M ctx, $2.00/$6.00	1M ctx, $1.25/$2.50
grok-4.20-0309-non-reasoning	2M ctx, $2.00/$6.00	1M ctx, $1.25/$2.50
grok-4.20-multi-agent-0309	2M ctx, $2.00/$6.00	1M ctx, $1.25/$2.50

Docs show grok-4.20 family received a price cut and context reduction. xBridge is stale.

Missing models

• grok-build-0.1 — fast coding model (256k ctx, $1.00/$2.00), released May 2026
• grok-imagine-image-quality — premium image model ($0.05/img)
• grok-imagine-video-1.5 — newer video model ($0.080/sec, supports 1080p)

All three should be added to AVAILABLE_MODELS, IMAGE_MODELS, or VIDEO_MODELS respectively, and documented in handle_grok_models() / handle_image_models().

Location

• xbridge_mcp/server.py — handle_grok_models() (L1213-1267), IMAGE_MODELS (L64-67), VIDEO_MODELS (L69-71), AVAILABLE_MODELS (L41-57)
• docs/wiki/Tools-Reference.md — model enum list (L20-21)

Acceptance criteria

• Pricing and context sizes match the current xAI models doc
• grok-build-0.1 added to AVAILABLE_MODELS
• grok-imagine-image-quality added to IMAGE_MODELS
• grok-imagine-video-1.5 added to VIDEO_MODELS
• grok-image-models handler updated to describe the new models
• handle_grok_models() pricing/context text matches docs
• Tools-Reference.md model enum list updated