tri pr preflight: land the parked glue net-zero, paid by dead-code deletions (the command does not run today) #11126

Goal

Land the parked glue of tri pr preflight so the command runs: the preflight fn in cli/tri/src/prcheck.rs (about 38 lines, as docs/now/2026-10-18-pr-preflight.md describes it), paid for by an equal or larger deletion of dead hand-written Rust in cli/tri/src, so the own-language budget nets to zero or less. The Queen's publisher side (tools/queen/publish.py, about 30 Python lines) stays out of this change.

Why

The spec specs/tri/pr/preflight.t27 (#10190) holds every rule and verdict, and two more rules are open in #11125 (own-language budget, developer-home paths). Today tri pr --help lists only landed and ready: the command that runs the spec does not exist, so nobody can run any of the rules before opening a pull request, and this loop replays them by hand. The note records why the glue is parked: check_budget refuses any net growth of hand-written foreign lines and the glue is not in tools/policy/foreign-exceptions.txt. The net-zero route needs no exception: PR #10538 landed 6 glue lines against a 7-line deletion of dead code, and the skill t27o records the recipe (cargo check -p tri lists never-used items; most have test callers, the truly dead ones are rare).

Boundary

• cli/tri/src/prcheck.rs
• cli/tri/src/main.rs
• specs/tri/pr/preflight.t27
• gen/rust/tri/pr/preflight.rs

User Scenarios

• Given a branch with a duplicated body, when tri pr preflight runs, then it prints the dupes rule as FAIL with the evidence and exits 1.
• Given a clean branch, when tri pr preflight --quick runs, then every rule prints PASS and the exit code is 0.
• Given output that is not the shape the spec reads, when it runs, then that rule is UNREAD and the exit code is 2.

Requirements

• FR-001: The glue MUST run sh -c step_cmd(k) for each step the spec says runs, frame the output as the spec documents, and print the spec's verdict; it MUST hold no rule.
• FR-002: The hand-written Rust added MUST be paid by an equal or larger deletion of dead code in cli/tri/src in the same diff (own-language net <= 0, no exception file).
• FR-003: The glue MUST loop to the spec's STEPS and RULES, so the steps added by spec(tri/pr): preflight -- the own-language budget from the merge base and the developer-home-path scan (Closes #11070) #11125 run without a glue change.

Success Criteria

• tri pr preflight --quick ends with exit 0 on a clean branch and exit 1 on a branch with a copied body.
• The own-language check reports net <= 0 for the PR.
• cargo check -p tri prints 0 new warnings for the glue.