Reshape AgentProvider so provider-specific assumptions stop leaking into
the generic layer. No change to what reaches sdkQuery() — same values,
different plumbing.
- QueryInput: opaque `continuation` replaces `sessionId` + `resumeAt`;
`systemContext.instructions` replaces ambiguous `systemPrompt`;
`mcpServers`, `env`, `additionalDirectories` move to `ProviderOptions`
at construction time.
- AgentProvider gains `isSessionInvalid(err)` and
`supportsNativeSlashCommands` so the poll-loop stops regex-matching
Claude error strings and gates passthrough slash commands per provider.
- ClaudeProvider owns `CLAUDE_CODE_AUTO_COMPACT_WINDOW` and the
stale-session regex internally.
- ProviderEvent.activity kept and documented as the liveness signal
(fires on every SDK message so the idle timer stays honest during
long tool runs); init carries `continuation` instead of `sessionId`.
- poll-loop drops mcpServers/env/systemPrompt from its config; admin
user id now passed explicitly.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
b63dd186df