Research + spike: cross-plugin agent roles

[mxriverlynn]

Research report on letting installed Han plugins contribute configuration that other Han plugins discover automatically, plus a spike that applies its recommendation to test-engineer.

Research

Report: docs/research/cross-plugin-config-discovery.md

Recommendation (O5): han-planning's team lists name roles, not agents. The moved agent's own description says which role it fills. A planning skill uses any available agent that claims a role, and skips the role when none does. This needs no hook and no new dependency.

A general startup-hook mechanism (O1) is deferred until a plugin needs to contribute something other than an agent.

Spike

• test-engineer moves from han-core into han-coding. Its description ends with Fills the han-planning test-planning role. han-coding's own skills now dispatch han-coding:test-engineer.
• New owned rule han-planning/references/agent-roles.md defines the test-planning role, how an agent claims it, and how a skill fills it or skips it with a one-line note.
• The four planning team lists (plan-a-feature, plan-implementation, plan-a-change, iterative-plan-review) name the test-planning role instead of han-core:test-engineer.
• test/agent-roles.bats fails when an agent claims an undefined role, or when no agent claims a defined role.
• Docs, indexes, the agent-placement rule in CONTRIBUTING, and the routing text in plan-synthesizer, junior-developer, and discussion-facilitator are updated.

Not done yet

• Live behavior check. No one has run a planning skill with han-coding installed and again without it.
• Release work. Version bumps and the CHANGELOG entry are not done.
• Historical docs. Plans and research under docs/ still say han-core:test-engineer, on purpose.