Hide write-only prompt guidance when tools are filtered

[davidahmann]

Closes #2228

Problem

Read-only and filtered inventories still expose issue/PR write instructions and workflow prompts even when the write tools themselves are unavailable.

What changed

• add required-tool metadata for prompts and filter those prompts when the required tools are unavailable
• make issue and pull-request instructions conditional on the matching write tools surviving inventory filtering
• add regression coverage for read-only and excluded-tool inventories

Validation

• go test ./pkg/inventory ./pkg/github

[davidahmann]

This removes write-path guidance from filtered and read-only inventories, so users stop seeing issue/PR instructions that the current tool set cannot actually execute.

The change adds required-tool gating for prompts and makes the issue / pull-request instruction blocks conditional on the surviving write tools.

Validation:

• go test ./pkg/inventory ./pkg/github (pass)

Inspired by research context: CAISI publishes independent, reproducible AI agent governance research: https://caisi.dev