Legacy manual-only Builder is undiscoverable during plan acceptance

[alvaroloes]

Problem

An existing project can show an enabled, healthy Builder and advertise /builder
as always available on demand, while approving a current Architect plan fails
with:

plan_acceptance_builder_not_configured

New projects seed Builder agents with both a GitHub command trigger and a manual
trigger. Older project rows can legitimately contain only the manual trigger.

findAgentDef currently falls back to the agent name only when triggers is not
an array. A valid trigger array with no command or handle therefore makes an
enabled legacy Builder undiscoverable, despite the UI's on-demand contract.

Expected behavior

• An enabled legacy agent with no explicit command binding is addressable by its
  name.
• A renamed agent with an explicit /builder binding remains addressable by that
  binding.
• An explicit remapping (for example, a Builder named builder bound to /ship)
  is not overridden by the name fallback.
• Approved plan execution can dispatch through a legacy manual-only Builder.

Acceptance

• Add focused unit coverage for fallback, explicit alias, and explicit remapping.
• Add an API integration regression that executes approved plan acceptance with
  a manual-only legacy Builder.
• Keep tenant/project/enabled filtering unchanged.