fix(plugin): prevent raw .ts import crash when esbuild transpilation fails

[ByteYue]

Problem

Fixes #500

When a TS plugin is installed but esbuild is unavailable or transpilation fails silently, the plugin discovery attempts to import() the raw .ts file at runtime, causing:

⚠ Plugin hot-digest/aggregate.ts: Unknown file extension ".ts"

Root Cause

1. transpilePluginTs() silently swallows all errors (bare catch {}), so if esbuild isn't available, no .js file is generated and no warning is shown
2. discoverPluginDir() sees the .ts file with no .js companion and attempts direct import(), which fails on production Node.js (only works under tsx in dev mode)

Changes

src/discovery.ts

• When a .ts plugin file has no compiled .js companion, skip the import and show an actionable warning:
  Plugin hot-digest/aggregate.ts: no compiled .js found. Run "opencli plugin update hot-digest" to re-transpile, or install esbuild.

src/plugin.ts

• Upgrade "esbuild not found" message from log.debug → log.warn so users know TS plugins won't work
• Log the outer catch error instead of silently swallowing it

Testing

• All 50 existing plugin tests pass
• Manual verification: removing .js from hot-digest plugin now shows a clear warning instead of crashing