Introduce a manifest (manifest.json) that indexes all cache entries,
replacing the old approach of scanning individual JSON files on disk.

Key changes:
- Session ID no longer auto-detected from parent process ID (unreliable
  with AI agents that spawn new shells per command)
- Three-tier session model: explicit INFRACOST_SESSION_ID (skips TTL),
  TERM_SESSION_ID fallback (respects TTL), and path+TTL only
- Manifest uses two maps: Entries (path hash → metadata) for O(1) path
  lookups, and Sessions (session ID → cache key) for O(1) session lookups
- Read/ReadLatest return *format.Output directly instead of *Entry wrapper
- Stale session mappings (>24h) pruned on save to prevent unbounded growth
- Manifest singleton moved from package-level var to Config struct

Replace Read/ReadLatest/Previous with three intent-based functions:
- ForPath(path): exact path lookup with TTL and freshness checks
- Latest(allowStale): most recent entry within TTL, no session logic
- ReadFile(path): raw JSON file, bypasses cache

Remove session tracking entirely (SessionID, TermSessionID, Sessions
map, session pruning). Terminal session IDs were unreliable across
different terminal emulators and agent environments. The Latest function
now handles the price command's temp directory case by simply returning
the most recent cached entry.