feat: fork selected agent text into a side pane

[oz-for-oss]

Closes #14610

Summary

• Adds a selected-text context-menu action for AI rich-content blocks: “Fork selection in new pane”.
• Builds fork parameters from the selected AI block text, preserving the source conversation/exchange boundary and opening the fork in a split pane.
• Adds focused terminal view tests for AI rich-content selection and ensures regular terminal text selection does not show the fork action.
• Includes small validation fixes needed for the required all-target/all-feature clippy command to pass in this checkout.

Validation

• ./script/format
• CARGO_BUILD_JOBS=1 cargo clippy --workspace --all-targets --all-features --tests -- -D warnings
• CARGO_BUILD_JOBS=1 cargo nextest run -p warp -E 'test(selected_ai_block_text_can_be_forked_in_new_pane) | test(regular_selected_text_context_menu_does_not_include_fork_selection)'

Notes

• The required GitHub context fetch script was not present at .agents/shared/scripts/fetch_github_context.py, so issue body/comments could not be fetched via the only supported mechanism for this run.