Add task plugin as an extension

Adds tools and skills for the task plugin into the extension framework
as a sibling package. `register()` wires up the tools `get_task_tree`
and `get_actionable_tasks` via `@requires_plugin("task")` so they
auto-route to hosts with the task plugin or raise a RuntimeError when
no host has it.

Both tools return a TypedDict (`_TaskTreeResult`,
`_ActionableTasksResult`) so the MCP SDK auto-generates an output_schema
and emits structured content alongside the text fallback. Error paths
raise exceptions rather than returning text/JSON-string error payloads.

Wire the sibling package into the build via a uv workspace: declare
`gerrit_mcp_server_task` as a workspace member and add it to the core
project's dependencies. Listing it as a plain dependency (rather than
relying on `uv sync --all-packages`) is what guarantees a plain
`uv sync` installs it, so its `gerrit_mcp_server.extensions` entry
point is discovered at startup and survives the `uv run` launch path.
The `gerrit-mcp-server = { workspace = true }` source is required for
the member to resolve its own dependency on the core package.

Update the build-and-run integration test to copy the new sibling
package into its clean-build temp dir; without it the workspace build
fails on the missing member.

Includes a gerrit-task-query skill and 15 unit tests covering
`_find_actionable` traversal and both registered tools.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Change-Id: I8c1bc9745962dbc7b355a1cb1220ffb07a141533