🐛 Require AC + Dependencies headings in plan prompt (closes #96)

[monkut]

Summary

Closes the contract gap between askcc plan and askcc develop reported in #96: plan could succeed without writing ## Acceptance Criteria / ## Dependencies headings, leaving develop's validate_issue_readiness to fail immediately.

Why

PLAN_AGENT_PROMPT items 1 and 2 used "replace any existing X" wording, which is ambiguous when no existing X is present. Item 2 explicitly handled the empty case ("If none, include the heading with 'None identified.'"); item 1 did not. A reasonable agent interpretation was "if no existing section, do nothing," leaving the body in a state develop rejects.

Changes

askcc/definitions.py — PLAN_AGENT_PROMPT issue-description-update section:

• Reworded items 1 and 2 to state the heading MUST be present, not just "replaced if found". Item 1 now also instructs renaming non-canonical headings (## Tasks, ## Requirements) to ## Acceptance Criteria.
• Added a new ## Post-Update Verification step: after editing, the agent re-reads the body and confirms both required headings are present (with a checklist item under AC), re-editing if either check fails. This converts the contract from "agent should remember to do it" to "agent verifies it before stopping."

Verification

• uv run pytest — 225 passed
• uv run poe check (ruff) — clean
• uv run poe typecheck (pyright) — 0 errors

Test plan

• Re-run askcc plan against a fresh issue with no ## Acceptance Criteria / ## Dependencies sections; confirm the resulting body passes validate_issue_readiness.
• Re-run askcc plan against an issue with a ## Tasks heading; confirm the agent renames it to ## Acceptance Criteria rather than leaving it.
• Re-run askcc plan against an issue that already has <!-- draft --> AC / Dependencies sections; confirm replacement still works.