Add Planning capability

[DouweM]

Summary

• Adds a Planning capability (AbstractCapability subclass) providing create_plan, update_task, and get_plan tools for structured task planning and tracking
• Plan state is per-run isolated via for_run() and dynamically injected into the system prompt via get_instructions()
• Task statuses: pending, in_progress, completed, skipped

Closes #39, relates to #65.

Test plan

• 32 tests covering all public API, tool logic, per-run isolation, dynamic instructions, edge cases (empty plan, invalid index, negative index)
• 100% branch coverage
• ruff lint and format clean
• pyright strict mode clean

🤖 Generated with Claude Code

[devin-ai-integration]

Devin Review found 1 potential issue.

View 3 additional findings in Devin Review.

[devin-ai-integration]

🔴 remove_task_impl corrupts parent_index when cascading removal of multiple indices

When remove_task_impl removes a parent task along with its subtasks (multiple indices removed at once), the parent_index adjustment loop on lines 206-209 produces incorrect results. The loop iterates through removed indices in ascending order, decrementing parent_index values above each removed_idx. However, after decrementing for the first removed_idx, the parent_index values are partially shifted, while subsequent removed_idx values remain in the original coordinate system. This causes comparisons like task.parent_index > removed_idx to fail prematurely.

Reproducer showing corrupted parent reference

tasks = [
    Task(description='A'),                       # 0
    Task(description='A-child', parent_index=0),  # 1
    Task(description='B'),                       # 2
    Task(description='B-child', parent_index=2),  # 3
]
remove_task_impl(tasks, 0)
# Expected: [B(parent=None), B-child(parent_index=0)]
# Actual:   [B(parent=None), B-child(parent_index=1)]  ← WRONG

B-child ends up pointing to index 1 (out of bounds), but B is at index 0. For a parent_index that was originally N, the code should subtract the count of removed indices below N, but the incremental approach only subtracts count - 1 because the last comparison fails.

Suggested change

	for removed_idx in sorted(indices_to_remove):
	for task in tasks:
	if task.parent_index is not None and task.parent_index > removed_idx:
	task.parent_index -= 1
	for task in tasks:
	if task.parent_index is not None:
	shift = sum(1 for ri in indices_to_remove if ri < task.parent_index)
	task.parent_index -= shift

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[DouweM]

Originally posted by @DouweM in #136 comment (PR was recreated)

Audit vs prior art: Planning

Worth adding now:

• Hierarchical subtasks: parent_index on Task + add_subtask tool (VStorm pydantic-ai-todo)
• insert_task and remove_task tools for mid-plan adjustment
• blocked status for dependency tracking

Follow-up opportunities:

• Persistent plans (file/DB), dependency graph, time estimates