ADR: Tekton-style explicit param wiring for workflow stages (remove ambient workflowParams / $(workflow.*) from AgentRun)

### Problem

Today (as of #169) workflow→stage parameter passing uses an **ambient model**: the AgentWorkflowRun controller stamps a `workflowParams` bag onto every stage AgentRun's spec, and agent-facing text resolves an ambient `$(workflow.*)` scope in addition to `$(agent.*)`.

This diverges from Tekton and creates an API-hygiene problem raised in review of #169 (comment on `agentrun_types.go:128`): `workflowParams` is a `preserve-unknown-fields` field on the **user-writable** AgentRun spec with no validation, so a standalone run can set arbitrary `workflowParams` and splice `$(workflow.anything)` into the Agent's prompt. (Not a hard escalation — the same user controls `spec.instructions` — but it's a leak of a controller-internal mechanism onto the user surface.)

### How Tekton does it

- `Task` declares `spec.params` (its own inputs); `Pipeline` declares `spec.params`.
- Each `pipelineTask` **explicitly wires** pipeline params into the task's declared params: `params: [{name: pathToContext, value: "$(params.context)"}]`.
- The `TaskRun` carries **only the Task's own declared params** (resolved). There is no ambient "pipeline params" bag on the TaskRun.
- Referenced Tasks (vs inline) must have params passed explicitly.
- Cross-stage data flows through **results** (`$(tasks.taskA.results.foo)`).

### Proposed direction

- `AgentWorkflowStage` gains `params: [{name: , value: "$(workflow.)"}]` (like `pipelineTask.params`).
- The workflow controller resolves the wiring and stamps the result as the stage AgentRun's **own** `spec.params` (agent params). **Remove `workflowParams` from `AgentRun` and the `$(workflow.*)` scope from agent-facing text.**
- `$(workflow.*)` survives only in the **guide**, rendered by the workflow controller (which holds the values) — never delivered to the AgentRun as a scope.
- Sets up **results/outputs** (`$(stages.X.results.Y)`) as the natural home for cross-stage data.

This removes the injection vector at the root (a standalone run then has only agent params, already validated against declarations) and aligns with Tekton's explicit-wiring model.

### Scope / impact
- CRD change (stage `params`), workflow-controller resolution, substitution scopes, and the #87 snapshot.
- Supersedes part of ADR 0009/0018's parameter-delivery model — needs an ADR.

### Related
- #169 (introduces the current ambient mechanism), #87 (snapshot), #180 (agent-definition immunity), ADR 0009, ADR 0018.

Start with agentrun_types.go:128 and the AgentWorkflowStage and AgentRun CRD definitions, then trace workflow-controller resolution and substitution scopes. Compare the #87 snapshot with #169, #180, ADR 0009, and ADR 0018; done means explicit stage wiring, no workflowParams on AgentRun, no workflow scope in agent text, and an updated ADR and snapshot.

Written by the indexing model from the issue text.