fix(bin): make heredoc assignments immune to bash-3.2 quote tracking

[idrishassan-ai]

Problem

macOS /bin/bash is 3.2.57, whose parser tracks quote state through a heredoc body nested inside $( ) command substitution.
A lone apostrophe in such a body breaks parsing of the entire script (issue #166):

unexpected EOF while looking for matching `''
syntax error: unexpected end of file

Modern bash (4+) parses the construct fine, so Linux CI never sees the failure.
PR #173 made bash -n pass by rewording the apostrophe away, but left the fragile VAR=$(cat <<EOF ...) construct in place: any future apostrophe in those bodies re-breaks every ship brief on macOS with no CI tripwire.

Fix

• bin/fm-brief.sh: assign the three ship-mode Definition-of-done heredocs with read -r -d '' DOD <<EOF ... EOF || true instead of $(cat <<EOF ...). bash 3.2 parses this regardless of body quoting. No brief wording changed.
• bin/fm-bootstrap.sh: convert the shim_body and cadence_body heredocs the same way (cadence_body keeps its quoted no-expansion delimiter). Extends the same hardening so the whole bin/ surface is free of the construct.
• tests/fm-brief.test.sh: bash -n now also runs explicitly under /bin/bash (the affected 3.2 interpreter on macOS), and a structural test refuses any non-comment $(cat << across bin/*.sh and bin/backends/*.sh, so the trap cannot silently return on Linux CI where bash -n cannot see it.
• docs: the bash-3.2 heredoc convention is recorded alongside the updated test guard.

Evidence (verified on /bin/bash 3.2.57, arm64-apple-darwin25)

• With an apostrophe injected into a DOD body, the old construct fails -n (exit 2, errors above); the new construct passes.
• All five generated briefs are byte-identical before vs after, built from the same FM_HOME: ship no-mistakes, ship direct-PR, ship local-only, --scout, --secondmate. The ship no-mistakes brief hashes to the same sha256 (142a8e66…) on both sides.
• Bootstrap-generated shim/cadence artifact contents are unchanged.
• Every script in bin/, bin/backends/, and tests/ passes /bin/bash -n under 3.2; tests/fm-brief.test.sh passes under both bash (5.x) and /bin/bash (3.2).
• Validated by the no-mistakes pipeline: review, test, document, and lint all green with no findings (push required this fork PR because the branch author has no upstream push access).

🤖 Generated with Claude Code