docs: add v2 to v3 migration guide, ADRs, and domain glossary

[jasoet]

Why

v3 carries 22 commits with BREAKING CHANGE footers and had no consumer-facing migration document — the single largest gap blocking the v3.0.0 release.

Worse, several breaks shipped in fix:-typed commits without footers, so semantic-release will never surface them:

• temporal.WorkflowManager.Close removed entirely (commit cadc208's footer omits it)
• rest.Client.HandleResponse unexported (6cc5af1)
• grpc.MountGatewayOnEcho now strips the base path; Start* return nil instead of http.ErrServerClosed; GetGRPCServer() returns nil after Stop
• server signal handling removed — consumers silently lose graceful termination, and nothing fails to compile to tell them

MIGRATION.md states each of these explicitly.

Accuracy

Every signature in the guide was verified against the code, not against the audit backlog's notes. That caught two places where the backlog described a planned API that did not ship:

backlog said	actually shipped
temporal.NewClient accepts/returns a ctx	NewClient(opts ...Option) (client.Client, error)
argo operations keep a namespace param	SubmitWorkflow(ctx, client, wf) — no namespace

Both are documented as built. All 14 packages covered, plus a closing section on telemetry changes that need no code edit but will move dashboards.

ADRs

Five decisions a future reader would otherwise have to reverse-engineer:

ADR	Decision
0001	Freeze v2, ship v3 as one big bang (module-path versioning is what makes it cheap)
0002	OTel config is injected, never serialized — a half-deserialized provider set is worse than none
0003	Constructor naming: New for the primary type, New<Thing> when there are several
0004	Selective de-leak — utility packages hide their dependency, SDK-integration packages don't
0005	grpc restarts, server doesn't; and the http.server.* attribute-set divergence

ADR 0003 resolves the "constructor naming split" backlog item as not a defect — retry.New returns a Config because a Config is retry's primary artifact. The rule already holds; renaming would have added breaks that make call sites worse.

CONTEXT.md pins the vocabulary those depend on (utility vs SDK-integration package, selective de-leak, escape hatch, convention contract, docs-of-record).

Also

• README.md claimed v2/v3 arrive "with minimal API changes". Corrected.
• INSTRUCTION.md gains MIGRATION.md, CONTEXT.md and docs/adr/ in Key Paths, plus a note to update the guide as breaks land rather than at release time.

All relative links in the new docs verified to resolve.