feat(slack): add Enterprise Grid workspace_users support (Phase 1)

[chaodu-agent]

Summary

Enhance the existing [slack] config section with Enterprise Grid workspace_users support for per-workspace trust scoping.

Current State

• [slack] section exists with basic allowed_users
• No per-workspace scoping for Enterprise Grid deployments
• Single-workspace Slack works fine

Target State

# Single-workspace (unchanged):
[slack]
allowed_users = ["U01ABCDEFGH", "U09XYZWVUTS"]

# Enterprise Grid — workspace-scoped:
[slack]
allowed_users = ["E0123456789"]   # enterprise_user.id (cross-workspace)

[slack.workspace_users]
T012345 = ["U01ABCDEFGH", "U01IJKLMNOP"]  # workspace-specific overrides
T067890 = ["U09XYZWVUTS"]

Tasks

• Wire Slack through the PlatformTrustConfigs registry (feat(trust): Phase 1 (slack) — L3 identity via shared gate #1363 — registry entry + redundant-but-matching gate_incoming in the adapter)
• Add workspace_users to [slack] config schema
• Implement SlackTrustConfig::decide() with workspace-scoped logic
• workspace_users = strict override (ignore allow_all_users for that workspace)
• Support team_id:user_id composite key format in base allowed_users fallback
• Receiver sets workspace_id = Some(team_id) for Grid deployments
• Prefer enterprise_user.id as sender_id when available
• Update docs and config.toml.example

Design Decisions (per ADR)

• workspace_users present for a workspace → strict per-workspace check (ignores allow_all_users)
• No workspace_users entry → fallback to base config
• Base fallback checks both plain sender_id AND composite "team_id:sender_id" format
• Rate-limit key includes workspace_id: (platform, workspace_id, sender_id)

Refs

• Umbrella: umbrella: Phase 1 trust migration — per-platform [section] for all adapters #1356
• ADR: docs(adr): revise identity-trust-none — three-layer architecture #1291

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