Make work-item artifact hosting fully provider-agnostic (Notion, Linear, …)

[tbrownio]

Problem

The skill system should be platform-agnostic, with a mechanism for configuring different artifact-hosting providers (e.g. Notion, Linear). PR #2 took the first step — publish-work-item.md now treats the host as a per-project provider: opt-in in CLAUDE.md, with GitHub-only marker comments as the default — but provider-specific references still leak through the shared skill text:

• claude/skills/do/SKILL.md — Step 0 names the notion skill and "Notion link" directly (Load step, plan stage, wrap-up upload).
• references/publish-work-item.md — frontmatter key example is notion:; the cross-link line format (**Notion:** <url>) is Notion-branded and load-bearing (pull greps for it).
• templates/CLAUDE.md — Work-item tracking YAML mixes generic (provider:) with Notion-specific keys (notion_data_source, properties).
• claude/skills/notion/SKILL.md — fine as a provider implementation, but it currently defines the publish/upload/pull contract implicitly rather than conforming to a documented one.

Desired end state

• A documented provider contract: any provider skill implements publish, upload, pull with the same inputs/outputs (local ./tmp/<id>/ in, page URL out; files column + durable raw copies; status updates).
• Shared skills (/do, publish-work-item.md, templates) reference only "the configured provider", never a specific one — a generic cross-link line (e.g. **Work item:** <url>) and a generic frontmatter key.
• Provider-specific config lives entirely in the provider skill + the project CLAUDE.md Work-item tracking section.
• Adding a Linear provider requires writing one new skill, zero edits to shared references.

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