fix: terminal full-width, flow line fade sync, CONTRIBUTING staleness

[johnnichev]

Summary

Three small but distinct fixes from the latest review pass.

Issue 1 — pip install terminal full-width on desktop (Image #32)

Problem: The terminal install component was capped at `max-width: 440px`, leaving it visibly narrower than the "Try the Builder" + "Read the Docs" button row directly below it on desktop. The misalignment looked unintentional.

Fix: Removed the `max-width: 440px` constraint. The terminal now spans 100% of the hero-content column. The hero grid (1fr 1fr split until the 1024px breakpoint) constrains the column itself, so the terminal doesn't span the entire viewport on wide screens — it spans the column where the buttons live, which is what aligns them.

```diff
.terminal-install {
width: 100%;

• max-width: 440px;
  ...
  }
  ```

Issue 2 — SVG flow lines lingering after nodes fade out (Image #33)

Root cause traced through the JS: In the hero flow scene transitions, nodes fade out via the CSS rule `transition: opacity 0.3s` when the JS sets `el.style.opacity = '0'`. Meanwhile, the SVG `` elements (and the `` pulses inside the same SVG) had no fade animation — they stayed fully opaque until the next scene's `buildScene()` cleared the SVG via `svg.textContent = ''`.

The visible result on a typical scene transition:

• t=0ms: Result toast hides, nodes start fading
• t=300ms: Nodes fully invisible
• t=300–400ms: Lines hang in the air alone with no nodes ← the bug
• t=400ms: `onDone` fires, next scene's buildScene clears the SVG
• t=400ms+: New scene's nodes and lines fade in

Fix (3 small changes):

1. Added an opacity transition to `.hf-svg`:
   ```css
   .hf-svg {
   ...existing rules...
   transition: opacity 0.3s var(--ease);
   }
   ```

2. In `playScene`'s "all done" handler, fade the SVG at the same time as the nodes:
   ```diff
   nodeEls.forEach(function(el) { el.style.opacity = '0'; });
   +svg.style.opacity = '0';
   delay(400, onDone);
   ```

3. In `buildScene` (called by the next scene), reset SVG opacity to 1 before drawing the new lines:
   ```diff
   nodesEl.textContent = '';
   svg.textContent = '';
   +svg.style.opacity = '1';
   ```

Lines and nodes now fade out together on the same 0.3s curve. No more lingering ghost lines.

Issue 3 — CONTRIBUTING.md staleness pass

The user asked me to "verify the contributing tab and update since I see that at least the version is already outdated." I caught the version stale fact and several others while I was there:

Stale	Was	Now
Header version	`v0.19.2`	`v0.20.1`
Header Python	`3.9+`	`3.9 – 3.13`
Header test status	`100%`	`95% coverage` (matches reality)
Project structure	`24 pre-built tools`	`33 pre-built tools`
Project structure	`61 numbered examples (01–61)`	`88 numbered examples`
Release example	`0.5.1` (ancient)	`0.20.2` (current minor + 1)
Test command	`python tests/test_framework.py`	`pytest tests/`
Provider test path	`tests/test_framework.py`	`tests/providers/test_your_provider.py`
Section header	`Adding RAG Features (New in v0.8.0!)`	`Adding RAG Features`

Verification of the new numbers:
```bash
$ find examples -maxdepth 1 -name '.py' | wc -l
88
$ grep -c '^@tool' src/selectools/toolbox/.py | awk -F: '{sum+=$2} END {print sum}'
33
```

The biggest substantive fix: rewrote the "Adding a New Tool" example. The old example used the legacy class-based API:

```python
return Tool(
name="your_tool",
description="...",
parameters=[
ToolParameter(name="param1", param_type=str, required=True),
...
],
function=your_tool_implementation,
)
```

Selectools has used the `@tool()` decorator pattern for many versions now, where the function signature and docstring are introspected automatically. The old example was actively misleading new contributors into using a deprecated API. The new example shows the modern decorator pattern with proper docstring conventions:

```python
from selectools import tool

@tool()
def your_tool(param1: str, param2: int = 10) -> str:
"""One-line description.

Args:
    param1: What this is for.
    param2: Optional. Default 10.

Returns:
    Description of the return value.
"""
return f"Result: {param1} with {param2}"

```

Plus a complete usage example showing how to wire the tool into an Agent.

Test plan

Desktop:

• pip install terminal aligns with the button row below it (same width)
• Visit https://selectools.dev and watch the hero flow animation cycle through scenes
• Verify lines fade out smoothly with the nodes (no lingering ghost lines)
• Verify lines fade in cleanly with the next scene's nodes

CONTRIBUTING.md:

• Open https://selectools.dev/CONTRIBUTING/ (or read the file directly)
• Verify version says v0.20.1
• Verify tool count says 33
• Verify example count says 88
• Verify the "Adding a New Tool" example uses `@tool()` decorator

Regression:

• Pre-commit hooks pass 14/14
• HTML structural balance OK
• `node --check` validates the JS
• No remaining `24 (tools|pre-built)`, `61 (examples|numbered)`, `v0.[0-9]`, `0.5.1`, `v0.8.0`, or `test_framework` strings in CONTRIBUTING.md (verified via grep)

What's NOT in this PR

• Full rewrite of the project structure block — only the genuinely stale numeric facts were fixed; the listed file names are still mostly accurate and a full architectural audit is out of scope for "the version is outdated"
• No CHANGELOG entry — these are doc fixes, not user-facing code
• No version bump