fix(item): surface a real diagnostic when claim's worktree resolution returns None

[getappz]

Summary

• item::claim's worktree_result collapses to None when item::get or resolve_target_branch fail even after the claim itself was Acquired — previously this silently omitted both worktree_path and worktree_error from the claim response, leaving the caller with no explanation.
• This adds a real diagnostic string ("item record could not be read back after claim") to worktree_error for that case, matching the sibling Some(Err(e)) arm's behavior.

This closes out the second of two gaps originally reported for item #153. The first gap — execute_work_impl (src/cli/work.rs) discarding worktree_error and printing a generic "bad git state?" guess — was already fixed independently by PR #550, with test coverage added in PR #554; this PR only needed to add the second, still-missing half.

Opened by claude-code for item #153 via agentflare.

Test plan

• cargo build -p agentflare clean
• cargo fmt --check clean
• cargo clippy --locked --workspace --all-targets --all-features -- -D warnings -A unsafe_code -A clippy::pedantic clean (matches CI's exact invocation)
• cargo test --bin agentflare mcp_server::tests::action_tests — 31/31 passing

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Reviewing files that changed from the base of the PR and between e801b19 and 21d3d9f.

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• src/mcp_server/item.rs

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