shell-quote treats # in claude_args values as comments, causing null values and AJV crash

**Describe the bug**

The action uses `shell-quote`'s `parse()` to process `claude_args`. When a flag value contains `#` (even if backslash-escaped), `shell-quote` treats it as a comment and returns a `{comment: "..."}` object instead of a string. The action's `.filter(arg => typeof arg === 'string')` then drops it, leaving the flag with no value, which the parser sets to `null`. This `null` propagates into the SDK config and triggers an AJV schema validation crash before any API calls are made.

**To Reproduce**

1. Create a workflow using `anthropics/claude-code-action@v1`
2. Set `claude_args` with a value containing `#` (note: `#` is preserved inside YAML literal blocks `|`):
```yaml
claude_args: |
--model claude-sonnet-4-5-20250929
--append-system-prompt \#\ Use\ best\ practices
```
3. Run the workflow
4. The action crashes with an AJV validation error at $0 cost

You can verify the parsing behavior locally with Node.js:

```js
const { parse } = require('shell-quote');

// Backslash escaping collapses \# to #, which shell-quote treats as a comment
parse('--append-system-prompt \\#\\ Use\\ best\\ practices')
// → ["--append-system-prompt", {comment: " Use best practices"}]

// The action filters out non-string objects:
// → ["--append-system-prompt"]
// → parser sets: {"append-system-prompt": null}

// Unescaped # is also treated as a comment
parse('--append-system-prompt # Use best practices')
// → ["--append-system-prompt", {comment: " Use best practices"}]

// Double-quoted values work correctly
parse('--append-system-prompt "# Use best practices"')
// → ["--append-system-prompt", "# Use best practices"]
```

**Expected behavior**

The action should parse `#` in flag values as literal text, not as a comment. At minimum, `null` values should be rejected with a clear error instead of being passed to the SDK.

**Workflow yml file**

```yaml
name: Repro AJV Crash
on: workflow_dispatch

jobs:
repro:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
- uses: anthropics/claude-code-action@v1
with:
anthropic_api_key: ${{ secrets.ANTHROPIC_API_KEY }}
prompt: "Say hello"
claude_args: |
--model claude-sonnet-4-5-20250929
--append-system-prompt \#\ Use\ best\ practices
```

**API Provider**

[x] Anthropic First-Party API (default)
[ ] AWS Bedrock
[ ] GCP Vertex

**Additional context**

The root cause is in `base-action/src/parse-sdk-options.ts`, `parseClaudeArgsToExtraArgs()`:

```typescript
const args = parseShellArgs(claudeArgs).filter(
(arg): arg is string => typeof arg === "string", // drops {comment: "..."} objects
);
```

When the loop reaches a `--flag` whose value was dropped, `nextArg` is `undefined`, so the parser falls into the boolean flag branch and sets `result[flag] = null`. The resulting config:

```json
{
"model": "claude-sonnet-4-5-20250929",
"append-system-prompt": null
}
```

This `null` triggers the AJV crash:
```
SDK execution error: 14 | depsCount: ${X},
15 | deps: ${Y}}`};var Tj={keyword:"dependencies",type:"object",schemaType:"object",...
error: Claude Code process exited with code 1
```

**Workaround:** Double-quote values containing `#`:
```yaml
# BROKEN — backslash escaping still results in # after shell-quote parses it
claude_args: |
--append-system-prompt \#\ Use\ best\ practices

# WORKS — double quotes prevent # from being treated as a comment
claude_args: |
--append-system-prompt "# Use best practices"
```

May be related to #947, #892, #852 which report the same AJV error signature.