detect-triggers refuses a round with no repository surface, so an investigation cannot be dispatched

Split out of #470, which routes a research request to a round instead of letting the lead answer it.

The gap

skills/herdr-teamlead/teamlead/triggers.py:451:

if not changes and not planned_lines and not (plan is not None and plan["cli_surface"]):
    raise UsageError("This round classifies nothing: ...")

An investigation-only round touches no repository surface: no diff, no paths it will add or change, no package sizes, no CLI surface. It has nothing to declare, so detect-triggers refuses it — and SKILL.md Step 5 makes that call mandatory before plan.

Concretely: "what's the best way to run integration tests against a Codex subscription" now correctly routes to a round (#470), reaches Step 5, and cannot get past it.

Why it isn't a one-liner

The guard is deliberate — #415 added it so a vacuous empty plan can't pass as "nothing fired." The fix needs a way to say "this round writes nothing" that a script can tell apart from silence, which is the same distinction .herdr/triggers.json already draws between an explicit [] and an omitted field.

Design choices to settle:

• a --planned field (writes_repository: false) plus a schema bump, or a detect-triggers flag
• whether the no-surface path is admissible only for read-only responsibilities (advisor / investigator / architect)
• what the four detected triggers mean for a round that writes nothing — presumably none can fire, which should be stated rather than inferred

Surfaces to update once chosen: triggers.py + its tests, references/specialists.md (declaration docs), SKILL.md Step 5, CHANGELOG.

Found by the Copilot review on #470.

Start at skills/herdr-teamlead/teamlead/triggers.py:451 and inspect the related trigger tests, then compare the explicit [] versus omitted-field behavior in .herdr/triggers.json. Resolve how an investigation-only round declares no repository surface and how triggers behave, then update triggers.py, its tests, references/specialists.md, SKILL.md Step 5, and CHANGELOG. Done means the Codex-subscription investigation can pass the mandatory detect-triggers step without allowing a vacuous plan.