docs: add Research & Discovery Flow

[dapi]

Goal

Add a governed Research & Discovery Flow for research tasks in marketing, product discovery, and software/technical discovery.

Scope

• Extend Task Routing with a research predicate and rerouting/handoff rules.
• Add a reusable lifecycle: intake → question framed → plan ready → evidence collection → synthesis → decision ready → handoff or explicit non-delivery outcome.
• Define artifact ownership, evidence provenance, confidence/limitations, decision disposition, and promotion into PRD, Epic, Feature, ADR, or no action.
• Add governed templates and indexes while keeping the template generic.

Acceptance criteria

• Research is distinct from delivery and cannot silently become implementation.
• Product discovery, market research, and technical discovery are supported as modes.
• Terminal states include validated, invalidated, inconclusive, parked/cancelled, and rerouted.
• Handoff preserves SSoT: durable facts move to the relevant canonical downstream owner.
• memory-bank lint and Go validator checks pass.

[dapi]

Completed by merged PRs #57 and #62. Closing after delivery of the Research & Discovery Flow.