1.0: Make pull and issue writes target, validate and merge the right thing #1327

Goal: pull and issue writes act on the intended repo and version, and refuse bad input before pushing or uploading.

• major: --repo names the target, but the base, fetch and default branch come from --remote. src/cmd/pr/write.rs:83-97, 377-405, 1005-1029.
• major: pr merge takes the version from Bobbin, not the knot. src/cmd/pr/write.rs:1312, 1473-1500.
• minor: Pull and comment bodies skip the length check, and create pushes first. src/cmd/pr/write.rs:440-451, 1884-1993, src/cmd/comment.rs:251, src/cmd/issue/write.rs:547.
• minor: pr edit --target --dry-run accepts a branch that does not exist. src/cmd/ticket_edit.rs:106-115, 240-249.
• minor: --new only overrides the other-target refusal. src/cmd/pr/write.rs:126-128, 583-619.
• minor: Stale comments describe the appview model. src/cmd/pr/write.rs:1722-1730, 1818-1827, src/clients/tangled/cob.rs:775-780.
• minor: pr merge has no --repo. src/cmd/pr/write.rs:1205, 1280.
• minor: The lease refusal says "round". src/cmd/mod.rs:461-465.
• minor: bobbin::patches is an unpaginated duplicate. src/clients/tangled/bobbin/mod.rs:94-105.
• minor: An unresolvable repo DID exits 0, 1 or 5 depending on the verb. src/cmd/ticket_state.rs:45-52.
• minor: Issue writers fail on an opener read they do not use. src/clients/tangled/cob.rs:389, 497-528.
• minor: Image warnings talk about a pull for issues and comments. src/cmd/images.rs:157-160, 298.
• minor: dest_position can rewrite a link in the alt text. src/cmd/images.rs:185-199.
• minor: pr comment --reply-to does not derive the pull from the parent. src/cmd/pr/write.rs:1889-1891.
• minor: The wrong-kind refusal suggests view. src/cmd/ticket_ref.rs:295-309.