pr detail: strip Claude Code attribution footer from PR descriptions

[ooloth]

Why

Every agent-generated PR description ends with a --- separator followed by a Generated by Claude Code (https://claude.ai/code/session_...) line. This appears on every PR and consumes 3 lines of visible space without adding review-relevant information.

Current state

The PR body is rendered verbatim via crate::markdown::from_str(raw_body) in ui/tui/src/render/mod.rs (line 709). The Generated by Claude Code footer and preceding --- separator are included in the rendered output on every agent-authored PR.

Ideal state

• Lines matching the pattern Generated by Claude Code (with or without a trailing URL) are stripped from the PR body before rendering
• The --- horizontal rule immediately preceding that line is also stripped if it would otherwise be the last visible content in the body
• All other body content is unaffected
• The stripping happens in the render path, not in the domain or fetch path (the raw body is preserved in the data model)

Out of scope

• Stripping attribution from comment bodies
• Removing other footers or boilerplate patterns

Starting points

• ui/tui/src/render/mod.rs — render_pr_detail() body rendering at lines 703–711

QA plan

1. Open any agent-generated PR in the TUI detail view (e.g. hub PR claude: remove external rust.md reference from Rust Conventions #160 or workflows/implement: filter claude stderr before forwarding to caller #159) — expect no Generated by Claude Code line or trailing --- separator at the bottom of the description
2. Press G to jump to the bottom of the description section — expect the last visible content to be ## Related links or similar substantive content, not the attribution footer
3. Open a PR with a manually written description that happens to end with --- — expect the --- to still render (the strip only applies when the line immediately follows the attribution pattern)
4. Open a PR with no attribution footer — expect no change to the rendering

Done when

The Generated by Claude Code attribution line and its preceding separator are absent from the rendered PR detail body.