fix(herdr-lab): place --session before the child-argv delimiter and refuse ambiguous delimiter shapes

[Amplify-Logic]

Intent

Fix bin/fm-herdr-lab.sh so 'run agent start ... -- ' places --session before the child-argv delimiter (Herdr consumes it instead of the child), reject ambiguous/unsafe delimiter shapes before invoking Herdr, and update the brief/doc wording that claimed the selector is always trailing.

What Changed

• bin/fm-herdr-lab.sh now splits run arguments on the first literal --: for agent start ... -- <child argv> it injects --session <name> before the delimiter so Herdr consumes it instead of the child process, while ordinary calls still get it appended; everything after the first -- (including further -- tokens) passes through to the child untouched.
• The run command refuses ambiguous or unsafe delimiter shapes before invoking Herdr: a delimiter on any command other than agent start, an option immediately before the delimiter (a value-taking option could swallow the injected --session), agent start with a delimiter but no child command, agent start without any delimiter, and a caller-supplied --session anywhere before the delimiter. tests/fm-herdr-lab.test.sh pins the accepted and refused shapes.
• bin/fm-brief.sh and docs/herdr-backend.md no longer claim the session selector is always trailing; they now describe the helper as placing --session on every call (before the child-argv delimiter for agent start), with the brief test updated to match.

Risk Assessment

✅ Low: The follow-up commit cleanly implements the requested first-delimiter-only contract with an exact regression test, removes no safety guard that pre-delimiter checks don't already cover, and leaves the header, docs, and tests consistent with the actual behavior.

Testing

Completed 1 recorded test check.

• Outcome: ⏭️ skipped across 1 run (23m22s)

Pipeline

Updates from git push no-mistakes

✅ **intent** - passed

✅ No issues found.

✅ **Rebase** - passed

✅ No issues found.

🔧 **Review** - 2 issues found → auto-fixed ✅

• ℹ️ bin/fm-herdr-lab.sh:181 - The multiple-delimiter refusal counts literal -- tokens inside the child argv, so a legitimate child command that itself contains -- (e.g. run <s> agent start probe -- npm run build -- --flag) is refused. This contradicts the header comment's claim that 'the opaque child argv itself passes through untouched' (line 26), and the error rationale ('could shift a lifecycle operation past the guard') does not apply to a -- after the first delimiter, since splitting and all guards key only off the first --. If the restriction is intentional fail-closed behavior, consider tightening the header wording; if not, count only pre-child delimiters.
• ℹ️ bin/fm-herdr-lab.sh:159 - The option-immediately-before-delimiter guard (refusing e.g. agent start --some-flag -- child) also rejects boolean flags that could never swallow the injected --session, and only inspects the single argument adjacent to the delimiter, so a hypothetical greedy multi-value Herdr option two positions back could still absorb the injected selector. As a shell-side heuristic this is a reasonable fail-closed best effort and is documented and tested as such; noting the boundary of the guarantee, no action needed.

🔧 Fix: only treat first -- as delimiter; child argv untouched
✅ Re-checked - no issues remain.

⏭️ **Test** - skipped

• 🚨 tests failed with exit code 1
• command -v tmux >/dev/null || { echo "tmux is required for e2e tests" >&2; exit 1; }; tmux -V; rc=0; for t in tests/*.test.sh; do echo "== $t =="; bash "$t" || rc=1; done; exit "$rc"

✅ **Document** - passed

✅ No issues found.

✅ **Lint** - passed

✅ No issues found.

✅ **Push** - passed

✅ No issues found.