Add org-member / assignable-assignee lookup tools

## Problem

`sentry-mcp` can search issues and update assignments, but it cannot safely resolve a human owner into a valid assignable Sentry actor in many MCP clients.

Today a workflow can do things like:
- find unresolved unassigned issues
- map a service to an owner outside Sentry
- call `update_issue`

But it cannot safely complete the last step unless it already knows a valid Sentry assignee such as `user:` or a verified `team:`. In practice this blocks safe assignment automation and pushes clients toward browser scraping, separate REST tokens, or guessing.

## Why this matters

A common workflow is:
1. find unassigned issues
2. resolve the correct owner from another source of truth
3. translate that human into a real assignable Sentry actor
4. assign the issue

Step 3 is the gap.

Without member or assignee discovery, MCP exposes write capability without the read capability needed to use it safely.

## Requested additions

Any of the following would solve the problem well:

1. Organization member lookup, ideally filterable by exact email
2. Team member lookup
3. Project-team lookup / access verification
4. A direct helper that resolves an email into an assignable actor for a given org or project
5. A helper that returns valid assignee candidates for a specific issue or project

## Minimum useful outcome

At minimum, expose enough data to move from:
- confirmed human identity

to:
- valid assignable `user:`
- or a proven `team:` that actually has access to the issue project

## Example blocked use case

An agent can find `is:unresolved is:unassigned` issues and determine the correct owner from a responsibility document, but cannot safely assign the issue because there is no MCP path to discover whether that owner is actually assignable in Sentry.

## Result

This would make issue assignment workflows first-class in MCP and remove the need for browser/session scraping or separate direct REST integrations just for assignee discovery.