fix(tri): gates preview reads the PR body the way the issue gate does (Closes #5744)

[dmitrii-f-t27]

Closes #5744

Stacked on #5732, which restructures the same code. The base moves to master once #5732 merges, and master is merged in then.

tri gates preview asked the required context check-linked-issue by matching issue-gate.yml's pattern against the pull request's whole body. The gate does not grep the body as written: its step "Check for linked issues in PR" first runs the body through an awk program (added under #3388) that drops every line inside a three-backtick fence and every line that opens with >, then greps the title and what is left. A PR whose only Closes #N sat in a fence or a quote read PASS in the preview and was refused by the gate: a false PASS on a required context.

Measured on #5732's head (ad43b60), the gate's own step under bash against the preview's old matcher (the cases used a real number; it is written #N here so this description links no issue):

body (title feat: x)	gate step	old preview
Closes #N inside a three-backtick fence	exit 1	PASS
> Closes #N	exit 1	PASS
This refs and #N on two lines	exit 1	PASS
Closes #N in prose (control)	exit 0	PASS

What changed

cli/tri/src/gates.rs:

• prose_filter reads the awk program out of issue-gate.yml at run time, from the job that posts check-linked-issue: the text between awk ' and the closing ') on the BODY_PROSE= line. The row runs it with the system awk on the body, fed as the gate feeds it (printf '%s\n'). The shape around the program is read too: exactly one BODY_PROSE assignment, the body piped straight into awk, the substitution closing right after the program, and the grep reading "$PR_TITLE" "$BODY_PROSE". Anything else reads UNAVAILABLE, never PASS, and so does an awk that cannot run.
• The pattern is matched one line at a time, as grep matches it. The regex over the whole text let \s* cross a newline (third row above). The commit proxy uses the same line rule, which is how l1-traceability.yml greps commits; it still reads PROXY, never PASS.
• The title is matched as written. gh pr view --json title,body is parsed into title and body apart instead of joined with --jq.

docs/BRANCH-PROTECTION.md: the table row for check-linked-issue said "the pull request title or body references an issue", the reading that produced this defect. It now says the title, or the body outside code fences and > quotes.

Tests (preview_tests, +7)

• a_reference_only_inside_a_fence_does_not_pass, a_reference_only_in_a_quote_does_not_pass, a_reference_in_plain_prose_still_passes (also prose after a fence, and a quote-shaped title), a_keyword_and_its_number_on_two_lines_are_not_a_reference.
• the_body_filter_is_read_out_of_the_gate_that_enforces_it: the program comes from the real issue-gate.yml and strips what the gate strips.
• a_gate_that_filters_or_greps_otherwise_has_no_filter_this_can_read: five altered gates read None (a pipe stage before or after awk, a grep of the raw body, another job, a second assignment).
• the_row_answers_as_the_gate_s_own_step_does: 13 cases, each run through the gate's own step under bash and through the row, both checked against a stated answer. The gate's quirks stay: a ~~~ fence, a four-space indent and prefs #N count for the gate, so they count for the preview. Whether the gate should strip those is not decided here.
• The new tests are anchored on closing braces; tri gates tests --gate reads 0 registered twice and 0 that do not run.

Mutations that restore part of the old behaviour, each run against the suite: grepping the raw body fails 3 tests, whole-text matching fails 2, filtering the title fails 2.

Verified locally

• cargo build -p tri --all-targets: ok
• cargo test -p tri -- --nocapture --test-threads=1: 841 passed, 0 failed (834 on fix(tri): gates preview asks the contexts the ruleset requires, parse-ratchet among them (Closes #5725) #5732's head, plus 7); cli/tri/skipwatch.py: no new runtime skips
• tri gates tests --gate, tri skill check, tri census pin --gate: exit 0; no pinned census moved
• python3 scripts/ci/test_now_gate_writes_nothing.py --tri ./target/debug/tri: ok
• python3 scripts/ci/test_required_gates_name_their_subject.py: ok; tri gates required: 5 claims, 2 hollow, 3 required, 0 unclaimed (unchanged)
• python3 tools/check_now_entry_shape.py --check-files docs/now/2026-10-03-tri-gates-preview-reads-the-body-the-issue-gate-reads.md: ok

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