feat: add session info panel with message count, context usage, cache stats, and skills

Add GET /bots/:bot_id/sessions/:session_id/info API endpoint that returns
per-session message count, latest input token usage with model context window,
aggregated KV cache hit rate, and skills invoked via use_skill tool calls.
Frontend Info tab in the right sidebar now displays this data in a compact
key-value layout with a context usage progress bar and clickable skill links.

db/queries/session_info.sql

@@ -0,0 +1,41 @@
+-- name: CountMessagesBySession :one
+SELECT COUNT(*)::bigint AS message_count
+FROM bot_history_messages
+WHERE session_id = sqlc.arg(session_id);
+
+-- name: GetLatestAssistantUsage :one
+SELECT
+  COALESCE((m.usage->>'inputTokens')::bigint, 0)::bigint AS input_tokens
+FROM bot_history_messages m
+WHERE m.session_id = sqlc.arg(session_id)
+  AND m.role = 'assistant'
+  AND m.usage IS NOT NULL
+ORDER BY m.created_at DESC
+LIMIT 1;
+
+-- name: GetSessionCacheStats :one
+SELECT
+  COALESCE(SUM((m.usage->>'inputTokens')::bigint), 0)::bigint AS total_input_tokens,
+  COALESCE(SUM((m.usage->'inputTokenDetails'->>'cacheReadTokens')::bigint), 0)::bigint AS cache_read_tokens,
+  COALESCE(SUM((m.usage->'inputTokenDetails'->>'cacheWriteTokens')::bigint), 0)::bigint AS cache_write_tokens
+FROM bot_history_messages m
+WHERE m.session_id = sqlc.arg(session_id)
+  AND m.usage IS NOT NULL;
+
+-- name: GetSessionUsedSkills :many
+SELECT DISTINCT
+  (part->'input'->>'skillName')::text AS skill_name
+FROM bot_history_messages m,
+  jsonb_array_elements(
+    CASE WHEN jsonb_typeof(m.content->'content') = 'array'
+         THEN m.content->'content'
+         ELSE '[]'::jsonb
+    END
+  ) AS part
+WHERE m.session_id = sqlc.arg(session_id)
+  AND m.role = 'assistant'
+  AND part->>'type' = 'tool-call'
+  AND part->>'toolName' = 'use_skill'
+  AND part->'input'->>'skillName' IS NOT NULL
+  AND part->'input'->>'skillName' != ''
+ORDER BY skill_name;