feat: add agent tools and sanitize tool names for Anthropic API

Add 8 new agent-callable tools (sessions.list/history/create/delete,
agents.list, message.send, cron.list/trigger) and sanitize tool names
at the API boundary (dots → underscores) to comply with Anthropic's
`^[a-zA-Z0-9_-]{1,128}` requirement. Reverse-maps sanitized names
back to internal names for hook callbacks and tool execution.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -6,6 +6,15 @@ All notable changes to Flynn are documented in this file.
 
 ### Added
 
+- **Agent Tools: sessions.\*** -- 4 new agent-callable tools (`sessions.list`,
+  `sessions.history`, `sessions.create`, `sessions.delete`) wrapping SessionManager
+  for runtime session management by the AI agent
+- **Agent Tools: agents.list** -- New tool exposing AgentConfigRegistry to the agent,
+  listing all registered agent configurations with tiers, profiles, and sandbox status
+- **Agent Tools: message.send** -- Cross-channel messaging tool allowing the agent to
+  proactively send messages to any connected channel (Telegram, Discord, Slack, etc.)
+- **Agent Tools: cron.\*** -- 2 new tools (`cron.list`, `cron.trigger`) for runtime
+  cron job management, allowing the agent to list and manually trigger scheduled jobs
 - **Web UI Dashboard (P7)** -- Full SPA control dashboard at the gateway web UI with
   four pages: Dashboard (health stats, channels, auto-refresh), Chat (session selector,
   streaming tool events, markdown rendering), Sessions (list, history viewer, delete),