Coder code patterns, refactor-first guidance, and structural task blocking

[wparad]

Summary

• Extends AgentRole::Coder's default system prompt (src/dispatcher/mod.rs) with explicit code-pattern guidance, following the same approach used for the Designer role's prompt extension:
  • Prefer early returns with if over if/else.
  • Avoid else if chains — use early returns or a match/switch instead.
  • Prefer default/fallback expressions for map lookups (e.g. dict[key] || default, map.get(key).unwrap_or(default)) over if/else value assignment.
• Adds refactor-first behavior to the Coder prompt: before implementing, the Coder checks whether refactoring existing code first would make the task trivial. If so, it creates a separate refactor ticket (gitzi_create_task) under the same epic, asks the human to confirm via gitzi_create_review_item, and — once accepted in a later run — blocks the original task on the refactor ticket instead of refactoring inline.
• Adds structural task blocking to support that flow:
  • Task gains a blocked_by: Vec<String> field (src/model/task.rs).
  • KanbanBoard::next_unblocked (src/dispatcher/board.rs) picks the highest-priority task in a column whose blockers have all reached Stage::Done; an unknown blocker ID fails closed (task stays blocked).
  • agent_pool.rs's task pick-up loop now calls next_unblocked instead of taking the column's first task unconditionally.
  • New gitzi_block_task(task_id, blocked_by) tool, wired through protocol.rs (schema), tools.rs (dispatch + ownership check), and coding_tools.rs (CodingLoop fallback). It moves the task back to the top of Prioritized and records the blocker list — unlike gitzi_park_task, which is a free-text note only, this is a structural link the pipeline enforces.

Test plan

• cargo build — succeeds
• cargo test --lib — all 90 tests pass, including new KanbanBoard::next_unblocked cases (no blockers, unfinished blocker, blocker reaches Done, unknown blocker ID)
• cargo clippy --lib — no warnings

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