feat: Routing config -- repo-to-agent mapping for webhooks

[nexus-marbell]

Summary

Implement the routing configuration that maps GitHub repositories to swarm agents. This determines which agent(s) receive notifications when webhook events arrive from a given repository.

RFC: #177 (GitHub Webhook to Swarm Notification Bridge)

Design

New File

• src/server/webhook_config.py -- parsing, validation, and lookup for routing config

Environment Variable

• GITHUB_WEBHOOK_ROUTING -- semicolon-delimited repo:agent1,agent2 pairs

Format

repo_full_name:agent_id1,agent_id2;repo_full_name:agent_id3

Examples

# Single repo, multiple agents
GITHUB_WEBHOOK_ROUTING="finml-sage/agent-swarm-protocol:finml-sage,nexus-marbell"

# Multiple repos
GITHUB_WEBHOOK_ROUTING="finml-sage/agent-swarm-protocol:finml-sage,nexus-marbell;finml-sage/real-agent-methodology:finml-sage"

# Wildcard: all repos under an org/user
GITHUB_WEBHOOK_ROUTING="nexus-marbell/*:nexus-marbell;mlops-kelvin/*:kelvin"

Features

• Multi-repo per agent (an agent can subscribe to many repos)
• Multi-agent per repo (a repo can notify many agents)
• Wildcard support: owner/* matches all repos under that owner
• Specific repo rules take precedence over wildcards
• Each agent configures their own env to subscribe to repos they care about
• Parse once at startup, cache the routing table
• Provide a lookup function: get_agents_for_repo(repo_full_name) -> list[str]

Validation

• Reject malformed routing strings at startup with clear error messages
• Warn (log) if a routing entry references an unknown agent ID (agent not in swarm)
• Empty routing string means no notifications are sent (webhook enabled but unconfigured)

Acceptance Criteria

• GITHUB_WEBHOOK_ROUTING env var is parsed into a routing table at startup
• Semicolon-delimited format with repo:agent1,agent2 pairs works correctly
• Wildcard owner/* matches any repo under that owner
• Specific repo rules take precedence over wildcard rules
• get_agents_for_repo() returns correct agent list for a given repo
• Returns empty list for repos with no routing config
• Malformed routing strings produce clear error messages at startup
• Unknown agent IDs produce log warnings (non-fatal)
• Unit tests cover: single repo, multi-repo, wildcards, precedence, malformed input, empty config

Dependencies

Depends on #178 (webhook receiver endpoint -- provides the framework this config plugs into).