🐛 Add verify-before-asserting-existence rule to plan prompt

[monkut]

Summary

Tightens PLAN_AGENT_PROMPT to prevent the plan agent from claiming a symbol does not exist when it actually does — the failure mode reported in #95.

Why

The current prompt's "Do not speculate about unopened code" guidance is too soft. In a recent run on kiconiaworks/kippo#7 the agent emitted an AddField step for display_in_project_report, even though the field is already defined at kippo/projects/models.py:206. If implemented as written, the migration would fail at apply time (column ... already exists) and the duplicate Python attribute could silently overwrite verbose_name/help_text.

Changes

askcc/definitions.py — PLAN_AGENT_PROMPT:

• New paragraph: "Verify before asserting existence" — for every symbol the plan proposes to add (field, function, class, setting, file), require a grep -n or Read-based confirmation of absence before writing "does not currently exist" or proposing AddField / "create new" steps.
• Tightened item 1 (Current state): require file:line citation for existing symbols and a negative-grep citation for any "missing" claim.

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 an issue mentioning an existing model field; confirm the plan does not include an AddField step for that field.
• Confirm the "Current state" section of generated plans cites file:line for referenced symbols.

Closes #95