feat: add /insights command with usage analytics and cost estimation

Inspired by Claude Code's /insights, adapted for Hermes Agent's multi-platform
architecture. Analyzes session history from state.db to produce comprehensive
usage insights.

Features:
- Overview stats: sessions, messages, tokens, estimated cost, active time
- Model breakdown: per-model sessions, tokens, and cost estimation
- Platform breakdown: CLI vs Telegram vs Discord etc. (unique to Hermes)
- Tool usage ranking: most-used tools with percentages
- Activity patterns: day-of-week chart, peak hours, streaks
- Notable sessions: longest, most messages, most tokens, most tool calls
- Cost estimation: real pricing data for 25+ models (OpenAI, Anthropic,
  DeepSeek, Google, Meta) with fuzzy model name matching
- Configurable time window: --days flag (default 30)
- Source filtering: --source flag to filter by platform

Three entry points:
- /insights slash command in CLI (supports --days and --source flags)
- /insights slash command in gateway (compact markdown format)
- hermes insights CLI subcommand (standalone)

Includes 56 tests covering pricing helpers, format helpers, empty DB,
populated DB with multi-platform data, filtering, formatting, and edge cases.

hermes_cli/commands.py

@@ -28,6 +28,7 @@ COMMANDS = {
     "/verbose": "Cycle tool progress display: off → new → all → verbose",
     "/compress": "Manually compress conversation context (flush memories + summarize)",
     "/usage": "Show token usage for the current session",
+    "/insights": "Show usage insights and analytics (last 30 days)",
     "/quit": "Exit the CLI (also: /exit, /q)",
 }