Spec ↔ issue linker

[androidand]

Spec ↔ issue linker

Why

A spec and its issue can originate from either side: an issue created first (by a
human or an orchestrator) that later gains a spec, or a spec authored first that
needs an issue. specsync must resolve "which issue is this change" robustly
across both, without inventing a parallel identity scheme.

Issue-first scaffolding is handled by the dedicated github-issue-pull change.
This change focuses on deterministic link resolution once local and tracker
artifacts exist.

What

Add a Linker abstraction that resolves a change to its issue ref by consulting,
in priority order:

1. the current git branch name (issue-linked branches encode the issue number),
2. the <!-- specsync:change=<slug> --> marker in issue bodies,
3. the local .specsync/ ref cache,
4. an external relation source (e.g. an MCP that knows issue↔branch↔repo links).

The first hit wins; the result is cached. This makes issue-first and spec-first
flows converge on the same link.

Scope

• Linker interface + chained resolver.
• Branch-name resolver (configurable pattern).
• Wire the resolver into the sync engine ahead of provider Find.

Tasks

Tasks: Spec ↔ issue linker

• Define the Linker interface and a chained resolver
• Implement branch-name resolver with a configurable pattern
• Implement marker + cache resolvers (refactor existing Find/cache logic behind it)
• Support spec-first creation when no link resolves