generateInstructions uses AvailableToolsets instead of EnabledToolsetIDs #1897

Bug Description

In pkg/inventory/instructions.go, the generateInstructions() function iterates over inv.AvailableToolsets() which returns all toolsets that have tools defined, rather than only the enabled toolsets based on the WithToolsets() configuration.

// Collect instructions from each enabled toolset
for _, toolset := range inv.AvailableToolsets() {  // <- BUG: should use enabled toolsets
    if toolset.InstructionsFunc != nil {
        if toolsetInstructions := toolset.InstructionsFunc(inv); toolsetInstructions != "" {
            instructions = append(instructions, toolsetInstructions)
        }
    }
}

Expected Behavior

When building an inventory with WithToolsets([]string{"repos"}), the generated instructions should only include instructions for the repos toolset.

Actual Behavior

Instructions for all toolsets (context, issues, pull_requests, discussions, projects, etc.) are included regardless of which toolsets are enabled.

Evidence

From mcp-server-diff action comparing v0.29.0 (base) vs v0.30.1 (branch) with repos toolset only:

• Base (v0.29.0): 1226 chars of instructions
• Branch (v0.30.1): 5886 chars of instructions

The v0.30.1 instructions incorrectly include:

• "Always call 'get_me' first..." (context toolset - not enabled)
• Discussions instructions (not enabled)
• Issues instructions (not enabled)
• Projects instructions (not enabled)

How to Reproduce

1. Update the mcp-server-diff action to v2.2.0 to see server instruction diffs in CI
2. Build an inventory with a single toolset: WithToolsets([]string{"repos"}).WithServerInstructions()
3. Check inv.Instructions() - it will contain instructions for all toolsets, not just repos

Suggested Fix

Use EnabledToolsetIDs() and look up toolset metadata, or add a new EnabledToolsets() method that returns []ToolsetMetadata for only enabled toolsets:

for _, toolsetID := range inv.EnabledToolsetIDs() {
    toolset := inv.GetToolsetMetadata(toolsetID)  // or similar
    if toolset.InstructionsFunc != nil {
        // ...
    }
}

Version

v0.30.1