FN-5780: inherit source GitHub tracking when refining tasks

Preserve GitHub linking intent when creating refinement tasks from completed work.

- Set refinement `githubTracking` from the source task state during `refineTask`
- Opt out refinements when the source task is unlinked or explicitly disabled
- Carry forward `enabled` and optional `repoOverride` for linked sources without copying source issue metadata
- Add store-level tests covering unlinked, disabled, linked, and issue-only source tracking states
- Document refinement GitHub tracking inheritance behavior in task-management docs
- Add a patch changeset for `@runfusion/fusion`

Files changed:
 .changeset/fn-5780-refine-github-tracking.md  |  5 ++
 docs/task-management.md                       |  1 +
 packages/core/src/__tests__/store-ops.test.ts | 71 +++++++++++++++++++++++++++
 packages/core/src/store.ts                    | 12 +++++
 4 files changed, 89 insertions(+)

Fusion-Task-Id: FN-5780
Fusion-Task-Lineage: 88c1ce01-6efd-48c1-bbdd-704a821055d7

docs/task-management.md

@@ -600,6 +600,7 @@ Behavior:
 - New title format: `Refinement: <source label>`
 - New task depends on source task
 - Created in `planning`
+- Refinement tasks inherit the source task's GitHub tracking state (unlinked sources opt out; linked sources inherit `enabled` and optional `repoOverride`, but never copy the source issue link)
 
 ## Archive and Restore