feat: add startup auth validation and workspace-isolated cache paths

[Flare576]

Summary

• Validates Slack tokens on startup via AuthTest() - server now fails fast with clear error message if tokens are invalid/expired
• Namespaces cache files by TeamID (e.g., T12345_users_cache.json) to prevent cross-workspace cache contamination when switching between Slack workspaces
• Logs authenticated workspace info on startup for debugging

Problem

Previously, if you had multiple Slack workspaces configured (e.g., one for work, one for personal), they would share the same cache files. This caused several issues:

1. Silent bad token: If workspace A's cache existed but workspace B's tokens were used, the server would start "successfully" (cache loaded!) but API calls would fail
2. Cache contamination: Switching between workspaces would overwrite each other's user/channel caches

Solution

1. Auth validation on startup: Call AuthTest() before loading cache. If auth fails, logger.Fatal() immediately with clear error message
2. TeamID-prefixed caches: Cache files are now namespaced by workspace: T12345_users_cache.json, T12345_channels_cache_v2.json

Testing

• With valid tokens: Server starts, logs "Authenticated to Slack" with team/user info
• With invalid tokens: Server fails immediately with "Authentication failed - check your Slack tokens"
• Multiple workspaces: Each workspace gets its own cache files

Depends On

• feat: add users_search tool for finding users by name/email #179 (feat-users-search) - this branch includes those changes, please merge feat: add users_search tool for finding users by name/email #179 first

[korotovsky]

@Flare576 Pls, rebase with master