feat: extract @libscope/connectors — third-party sync adapters

[RobertLD]

Summary

Part of #488. Extract src/connectors/ into a standalone @libscope/connectors package, and fix the inward coupling where src/core/scheduler.ts currently imports connector modules directly.

Problem / Motivation

Connector code (Notion, Slack, Confluence, OneNote, Obsidian) has no circular dependencies with core — but src/core/scheduler.ts imports connectors directly, creating an inward dep that prevents core from being independent. This coupling needs to be inverted before connectors can be a separate package.

Proposed Solution

1. Move src/connectors/ into packages/connectors/
2. Move src/core/scheduler.ts into packages/connectors/ (it's logically a connectors concern — scheduling syncs)
3. Update scheduler to accept connector functions as injected dependencies rather than importing them directly
4. @libscope/connectors depends on @libscope/core for indexing; @libscope/core has zero imports from connectors

Acceptance Criteria

• @libscope/connectors builds independently; zero imports from @libscope/connectors exist in @libscope/core
• src/core/scheduler.ts is moved to packages/connectors/ and no longer lives in the core package
• The scheduler's connector dependencies are injected (passed in) rather than statically imported
• All connector sync functions (notion, slack, confluence, onenote, obsidian, docs) work correctly end-to-end
• Existing CLI connector commands (libscope connect, libscope sync) pass integration tests

Out of Scope

• Adding new connectors
• Changing connector authentication flows

Technical Notes

• Connectors: notion.ts, slack.ts, confluence.ts, onenote.ts, obsidian.ts, docs.ts
• The only inward dep to fix is src/core/scheduler.ts → connectors. All other connector deps are outward (connectors → core/indexing)
• src/connectors/index.ts currently exports connector config functions and some DB helpers — review what belongs in connectors vs core