Enforce a configurable rolling image window before model requests

[darkmatter2222]

Screenshot-heavy sessions accumulate raw images and can exceed a model's per-request image limit. This change defaults Master Goal to the newest image across each outgoing conversation, with native plugin configuration for other limits.

Configuration:

{"plugins":[{"package":"@darkmatter2222/opencode-mastergoal","options":{"imageWindow":1}}]}

Keep the existing package path when editing an installed registration. Explicit options override MASTERGOAL_IMAGE_WINDOW, then default 1. Zero excludes all raw images. Invalid selected values reject startup. Installer upgrades preserve options.

The existing context/compaction callbacks filter request messages before goal injection; title/generate hooks cover auxiliary session requests. Filtering handles canonical Media.Asset image parts (bytes, base64, URL, provider reference) and image MIME files nested in tool-result content. It preserves text, non-image files, tool pairs and source history, with structural sharing and linear traversal. No lifecycle changes or dependency upgrades; @opencode/plugin stays at 2.0.22.

Validation:

• npm run verify: 117 tests and 20/20 deterministic scenarios passed.
• npm run test:install: packaged local/global installation and uninstall passed.
• Actual OpenCode 2.0.22 HTTP provider capture: 100 stored image-bearing prompts produced [1,1,1] images across three requests with default configuration; limits 4 and 0 produced [4,4,4] and [0,0,0].
• All 100 screenshot attachments remained in exported persistent history. A distinct newest PNG was verified in the default provider payload.
• Tests include 1,000 growing turns, global ordering, tool screenshots, non-image preservation, frozen inputs, strict settings, auxiliary hooks, existing goal injection and installer option preservation.

Exact files changed:

• src/image-window.ts, src/server.ts, src/install.ts, src/commands.ts
• test/image-window.test.mjs, test/adapters.test.mjs, test/commands.test.mjs
• scripts/host-canary-v2.mjs
• README.md, docs/architecture.md, docs/validation.md

Scope limits: OpenCode offers no final hook priority; another plugin inserting images after this filter can invalidate its ceiling. Opaque provider-managed history and calls outside session hooks are not inspected. Provider capture uses a local OpenAI-compatible fake, not live vLLM. Automatic compaction is disabled in the capture fixture; auxiliary filtering is tested at the official hook boundary. Full details and reproducible commands are in docs/validation.md.