feat: cache hit observability, configurable TTL, optimized marker stripping

[binhex]

What's Changed

Three improvements to the opencode-go cache extension (v0.4.0):

📊 Cache hit observability (TUI footer)

• New message_end hook parses Pi's normalized usage to extract cache hit data
• Footer shows live cumulative (CU) and turn-based (TB) cache hit percentages
  (e.g. opencode-go-cache: CU98% TB100%)
• Cumulative average tracks total savings over the session; per-message shows
  how much of the latest response came from cache
• Verified working live against the real gateway — cacheRead > 0 confirmed

⚙️ Configurable TTL (PI_OPENCODE_CACHE_TTL)

• Env var replaces the hardcoded "1h" default
• Accepts values like "30m", "2h", or "0"/"off" to omit ttl entirely
• Invalid values warn and fall back to "1h"
• Tightened regex to reject nonsensical zero-TTL values ("0h", "00m")

⚡ Optimized marker stripping

• Replaced recursive tree walk with non-recursive targeted clear
• Checks only known marker surfaces (content parts, system blocks, tool defs)
• Added nested tool_result content clearing for round-tripped markers
• ~25 fewer lines, same correctness

🌐 OpenCode Zen support

• Provider gate changed from single "opencode-go" to ["opencode-go", "opencode-zen"]
• Extension now works with both Go and Zen gateways (same Anthropic-standard cache fields)

Other

• README updated with new features and verified-live results
• QA configs added: ESLint, Prettier, pre-commit
• Code-reviewed by two independent models, all issues fixed