Workflow template variables render literally in send_message despite inputs: declaration + --inputs JSON

## Describe the bug

Workflow template variables (`{{gate_name}}`, `{{gate_description}}`) render as **literal strings** in `send_message` step output. The workflow engine does not interpolate `{{variable}}` references, regardless of whether inputs are declared in the workflow YAML and/or passed via `buzz workflows trigger --inputs ''`.

## Steps to reproduce

### 1. Create a workflow with template variables

```yaml
name: trent-gate-approval
description: Replaces trent-gate-pusher 60s poller.
trigger:
on: webhook
inputs:
- key: gate_name
required: true
- key: gate_description
required: true
steps:
- id: gate_notification
action: send_message
text: "Gate approval required: {{gate_name}} - {{gate_description}}"
- id: trent_approval
action: request_approval
from:
message: "Approve gate: {{gate_name}}"
- id: result
action: send_message
text: "Gate {{gate_name}} resolved by Trent."
```

Workflow ID: `2524dcb8-b67d-4146-9514-0d0303a75503`
Channel: `#command-center` (`fd688946-bf0e-4a70-bc9d-f51bd19736aa`)
Relay: `wss://suppvault.communities.buzz.xyz`

### 2. Trigger it with inputs

```bash
buzz workflows trigger \
--workflow 2524dcb8-b67d-4146-9514-0d0303a75503 \
--inputs '{"gate_name":"INTERPOLATION-TEST","gate_description":"Testing if explicit inputs declaration fixes template interpolation"}'
```

### 3. Observe the message that lands in-channel

The `send_message` step posts: `Gate approval required: {{gate_name}} - {{gate_description}}`

The template variables are **not** replaced with the input values.

## Expected behavior

`send_message` should post: `Gate approval required: INTERPOLATION-TEST - Testing if explicit inputs declaration fixes template interpolation`

At minimum, simple `{{key}}` substitution from `--inputs` JSON should work. Full mustache conditionals (`{{#key}}...{{/key}}`) are nice-to-have, not blocking.

## Version and platform

- Buzz version: Desktop 0.5.5
- Relay: `wss://suppvault.communities.buzz.xyz`
- OS: macOS 26.5.2

## Logs / additional context

### 4 trigger attempts — all show literal template vars

| Run | Event ID | Content (verbatim) |
|-----|----------|---------------------|
| 1 | `3bb2dd19...` | `Gate approval required: {{gate_name}} - {{gate_description}}` |
| 2 | `d667d286...` | `Gate approval required: {{gate_name}} - {{gate_description}}` |
| 3 | `dcc841ba...` | `Gate approval required: {{gate_name}} - {{gate_description}}` |
| 4 (post-update) | `a5ea5fec...` | `Gate approval required: {{gate_name}} - {{gate_description}}` |

All confirmed via `buzz messages get --channel --limit N`.

### What was tested to rule out user error

1. **Without explicit `inputs:` declaration** — same result (literal template vars)
2. **With explicit `inputs:` declaration** in workflow YAML (added `inputs:` block with `key`/`required` fields) — same result after workflow update
3. **JSON input format** — `--inputs` receives valid JSON with keys matching template var names
4. **Direct `buzz messages send`** — works correctly (pre-formatted message arrives as written), confirming the issue is specific to the workflow `send_message` step, not the message pipeline

### Separate from request_approval defect

This is independent of the `request_approval` silent-no-op bug (filed separately). Template interpolation affects `send_message` text even in workflows that don't use `request_approval`.

Start at the workflow engine's send_message step and the buzz workflows trigger --inputs entry point; reproduce the issue with the YAML workflow and JSON inputs shown. Trace how gate_name and gate_description reach message text rendering. Done means simple {{key}} references are replaced with the supplied values in send_message output, with regression coverage for the reported case.

Written by the indexing model from the issue text.