Feature: per-toolset (or per-tool) read-only mode instead of global GITHUB_READ_ONLY

首先跟踪服务器如何处理 GITHUB_READ_ONLY 和现有的 GITHUB_TOOLSETS 配置，然后确定写入工具在哪里进行分发。将按 toolset 和按 tool 提出的 scope 与当前配置行为进行比较。完成的标准是：选定的写入操作会被拒绝，同时读取操作和明确允许的写入操作继续正常工作，并且为所选配置提供覆盖。

Feature request

GITHUB_READ_ONLY is currently all-or-nothing: when set, the whole server rejects every write operation. There is no way to keep some domains writable while others stay read-only.

Use case

Running the server as a local coding-agent tool with a single GitHub token, I want a common "safe by default" posture:

• Reads everywhere (repos, issues, pull requests) allowed without human confirmation
• Writes (merge PR, close/label issues, push comments) gated behind explicit user approval

Today the only way to approximate this is to launch two full server instances — one with GITHUB_READ_ONLY=1 and one without — and rely on agent-side conventions to route writes to the second instance. That is fragile because nothing on the server side prevents an agent from calling write tools on the writable instance, and it doubles the tool surface / process count.

Proposed solution

Any of the following would fix it:

1. Per-toolset read-only, e.g. GITHUB_READ_ONLY_TOOLSETS=issues,pull_requests (writes rejected only for the listed toolsets), or
2. Per-tool read-only overrides, e.g. a GITHUB_READ_ONLY_TOOLS=merge_pull_request,create_issue deny list, or
3. A "write-confirm" layer that rejects write tools unless an opt-in env var for that specific call is present.

Option 1 seems the most consistent with the existing GITHUB_TOOLSETS design.

Alternatives considered

• Token scoping (fine-grained PAT without write scopes): does not help, because the same token is also expected to perform approved writes.
• Dual-instance setup: works only as a convention, not enforcement (described above).