docs: add SDK & embedding developer-experience plan (#2525)

Unifies the scattered roadmap threads (multi-language SDKs, local testing, AI
generation, embedded-v1, pipelines-in-code) into one cohesive DX plan for the two
developers who build ON Conduit: the plugin author and the embedder. Adds
acceptance criteria, edge-case mitigation, a lifecycle-stage matrix, and a clear
docs plan.

Highlights: one versioned acceptance suite defines "a connector" across all
languages; a first-class `conduit connector test` local loop with golden record
fixtures; grounded AI generation (plain language / --from-openapi / --from-docs)
that must pass acceptance before it ships; a real semver-stable Go embedding API
(today's Serve is CLI-shaped and os.Exits); a fluent pipelines-in-code builder
that round-trips with YAML; libconduit + Python/Node bindings carrying the same
ConduitError codes and lifecycle verbs. Measured targets: <30-min first connector,
<15-min first embedded pipeline, both CI-timed.

Maps into existing phases (no new phase); makes the threads one workstream.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: devarismeroxa