Hide write-oriented prompts and instructions when write tools are filtered

先阅读 pkg/inventory/instructions.go 和 pkg/inventory/prompts.go，然后跟踪过滤过程进入 pkg/github/toolset_instructions.go。复现一个只读或 exclude-tool inventory，并比较 AvailableTools、AvailablePrompts 和生成的 instructions。当过滤后的 inventory 不再暴露暗示不可用写入操作的 prompts 或 instruction 文本时，即表示完成；有关已文档化的行为，请参阅 docs/toolsets-and-icons.md。

Problem

Tool filtering can remove write capabilities while prompts and server instructions continue to advertise those same actions. In read-only or explicitly filtered configurations, the server can still expose prompts that instruct the client to create issues, assign Copilot, or open pull requests.

Why now

This server builds instructions and prompts dynamically as part of its safety surface. If filtered capabilities remain documented as available, clients receive contradictory guidance and can plan actions the server has already hidden.

Version under test

• Commit: b1575edfefde09e3cf7c805aea79a92131271659 (origin/main on 2026-03-17)
• Runtime: Go 1.25.7, macOS darwin/arm64

Minimal repro

1. Build an inventory in read-only mode or with write tools excluded.
2. Inspect AvailableTools, AvailablePrompts, and generated server instructions.
3. Compare the filtered tool set with the exposed prompts/instruction text.

Expected behavior

If a write tool is hidden by policy, related prompts and write-oriented instructions should disappear or clearly mark the capability as unavailable.

Actual behavior

Prompt availability is filtered only by toolset/feature flags, and instruction generation does not verify that the referenced write tools survived read-only or exclude-tool filtering.

Code paths

• pkg/inventory/instructions.go
• pkg/inventory/prompts.go
• pkg/github/toolset_instructions.go
• docs/toolsets-and-icons.md

Validation target

Read-only and explicitly filtered inventories should not expose prompts or instruction text that imply unavailable write actions.