fix(item): don't open a redundant PR when item done runs after the PR already merged

[getappz]

Summary

• push_and_open_pr() (src/worktree.rs) unconditionally called github::pulls::create() for the item's branch, with no check for an existing PR. GitHub's API only rejects a duplicate while the existing PR is still open -- once it's merged (or manually closed), a second PR against the same branch is perfectly legal to create.
• Observed live 2026-07-25: PR fix(worktree): never let doctor reclaim delete the main worktree #327 was created manually and merged, then item done on the same item auto-opened redundant PR fix: doctor reclaim can delete the main/canonical worktree, cascading to all linked worktrees #328 against the same branch (had to be closed manually).
• Adds pulls::find_existing(client, repo, branch) -- lists PRs with state=all and finds one whose head.ref matches the branch -- and calls it from push_and_open_pr before create. If a match is found (open, merged, or manually closed), reuses its URL instead of opening a duplicate.
• Lookup failures are soft-failed the same way the rest of this function already is: logged, then falls through to create -- a rare duplicate is a smaller harm than silently never opening a PR on a transient lookup error.

Test plan

• cargo test --bin agentflare github::pulls:: -- 14 passed, 0 failed (incl. 2 new find_existing tests)
• cargo clippy --bin agentflare --all-targets --all-features -- -D warnings -A unsafe_code -A clippy::pedantic -- clean
• cargo fmt -- clean

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Review details

⚙️ Run configuration

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Run ID: 67f1190d-de9c-473a-b61a-44439ed7aea9

📥 Commits

Reviewing files that changed from the base of the PR and between e88cccd and 3892fd6.

📒 Files selected for processing (2)

• src/github/pulls.rs
• src/worktree.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch task/349

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