Add unit tests for ObjectivePlugin

`ObjectivePlugin` (https://github.com/fuseraft/fuseraft-cli/blob/main/src/Infrastructure/Plugins/ObjectivePlugin.cs) has no test file — there is no `ObjectivePluginTests.cs` under `tests/FuseraftCli.Tests/`.

It's a thin agent-facing wrapper around `ObjectiveManager` (https://github.com/fuseraft/fuseraft-cli/blob/main/src/Infrastructure/Objectives/ObjectiveManager.cs) that exposes long-horizon objective tracking: `CreateAsync`, `ReadAsync`, `UpdateAsync`, `ListAsync`, `LinkTaskAsync`. Same shape as `TodoPlugin` (which already has `TodoPluginTests.cs`), so that file is a solid template to copy the setup pattern from.

Good first areas to cover:
- `CreateAsync` rejects an empty title (`PluginResult.Error`)
- `CreateAsync` with a comma-separated `tasks` string populates remaining tasks
- `ReadAsync` on an unknown ID returns `PluginResult.NotFound`
- `UpdateAsync` only changes the fields passed (title/description/status), leaving others intact
- `ListAsync` with a `status` filter only returns matching objectives, and returns `NotFound` when nothing matches
- `LinkTaskAsync` with `completed: true` moves a task into `CompletedTasks` and recomputes `PercentComplete`; with `completed: false` it's a no-op add to `RemainingTasks`

`ObjectiveManager` takes an `ObjectiveStore` — check how `TodoPluginTests.cs` constructs its manager/store for the pattern to follow (likely a temp-directory-backed store).

Start with src/Infrastructure/Plugins/ObjectivePlugin.cs and src/Infrastructure/Objectives/ObjectiveManager.cs, then use tests/FuseraftCli.Tests/TodoPluginTests.cs as the setup template. Construct the manager and ObjectiveStore using the established pattern, add coverage for the listed create, read, update, list, and link-task cases, and run the test project to confirm all behaviors pass.

Written by the indexing model from the issue text.