fix(prompts): add PR already-reviewed guard and reduce redundant issue comments

[monkut]

Closes #100

Summary

• Adds a Pre-review dedup block to REVIEWPR_AGENT_PROMPT (placed before Pre-merge guard) that short-circuits when ellen-goc's last substantial review postdates the PR's most recent commit — no review, no linked-issue comment.
• Removes the unconditional Also post a brief summary on the linked issue line from REVIEWPR_AGENT_PROMPT. The PR review is now the canonical record.
• Replaces the unconditional Comment on the issue summarizing what was implemented bullet in DEVELOP_AGENT_PROMPT with a two-branch instruction: comment when the session opened a new PR; skip on follow-up commits (the PR description update is sufficient).
• Adds Mermaid label safety guidance to DEVELOP_AGENT_PROMPT so labels containing /, \, (, ), |, :, etc. get quoted (B["/simplify, commit, push"]) instead of being parsed as parallelogram/trapezoid shape syntax — prevents the "Lexical error / Unrecognized text" rendering failure that broke the original PR description.
• Adds TestReviewprPromptDedupGuard, TestReviewprPromptNoLinkedIssueComment, TestDevelopPromptConditionalIssueComment, and TestDevelopPromptMermaidLabelSafety to tests/test_askcc.py using the existing literal-substring assertion style.

Verification

• uv run pytest — passed (236 tests, +11 new)
• uv run ruff check — passed (no issues)
• uv run pyright — passed (0 errors, 0 warnings)

Key Flows

flowchart TD
    A[pr-review triggered] --> B[Pre-review: read diff, gh pr checkout, run tests]
    B --> C{Pre-review dedup:<br/>last_review &gt;= last_commit?}
    C -- yes --> D[Skip entirely:<br/>no review, no issue comment]
    C -- no --> E[Pre-merge guard:<br/>CHANGES_REQUESTED check]
    E --> F[Definition of Done checklist]
    F --> G[gh pr review --approve / --request-changes]

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flowchart TD
    A[develop completes] --> B["/simplify, commit, push"]
    B --> C{New PR opened<br/>this session?}
    C -- yes --> D[Open PR + comment on issue:<br/>summary of what was implemented]
    C -- no --> E[Update existing PR description:<br/>skip issue comment]

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Test plan

• TestReviewprPromptDedupGuard asserts Pre-review dedup heading, gh pr view --json reviews,commits query fragments (sort_by(.committedDate), .author.login == "ellen-goc", (.body|length > 50), --json reviews,commits), skip entirely, and dedup-before-merge-guard ordering.
• TestReviewprPromptNoLinkedIssueComment asserts the removed line is absent.
• TestDevelopPromptConditionalIssueComment asserts the unconditional bullet is gone and both conditional branches (If this session opened a new PR, PR already existed, PR description update is sufficient) are present.
• TestDevelopPromptMermaidLabelSafety asserts the Mermaid label safety heading, the quoted B["/simplify, commit, push"] example, and the parallelogram/trapezoid shape syntax warning.
• uv run pytest / uv run ruff check / uv run pyright all green locally.
• Runtime effect verified by observing weyucou/sops PRs on the next review cycle after merge (matches the precedent set by issues review action: should not merge when PR has CHANGES_REQUESTED #86 / develop action: update test plan checklist in PR description after PR creation #88 / develop and fix-ci prompts: never modify linter (ruff, pyright, etc.) configuration without explicit request #89, which also relied on prompt-content tests + post-merge observation).