Pluggable work providers

[androidand]

Pluggable work providers

Why

specsync currently shells out to gh directly. To support self-hosted trackers
and orchestrators that already own issue lifecycle, the projection target must be
an interface, not a hardcoded binary.

What

Introduce a WorkProvider interface (already present: Name, Push, Find)
and ship multiple implementations behind it:

• github — the current gh CLI provider (default, zero infra).
• mcp — an MCP client that delegates issue create/update/link to an external
  work-management MCP server, reusing its repo-relation knowledge.
• vikunja / plane — self-hosted providers (later).

The provider is selected by flag/config; the core engine is unchanged.

Scope

• Provider selection flag (-provider) and config resolution.
• provider/mcp client implementation.
• Capability interfaces (comments, sub-items, custom fields) detected by type
  assertion so a minimal provider need not implement everything.

Tasks

Tasks: Pluggable work providers

• Extract the provider selection into a -provider flag (default: github)
• Define capability interfaces: CommentCapable, SubItemCapable, CustomFieldCapable
• Implement provider/mcp as an MCP client (stdio + HTTP transports)
• Document the provider contract in the README
• Add a fake provider for end-to-end tests of the sync engine