[bug-hunter] filelog_otel multiline regex vars render invalid YAML when unquoted

## Impact
Users configuring `filelog_otel` multiline regex patterns can produce an invalid rendered input config, which prevents the input from starting. This is user-impacting because multiline regex is a documented setting, and valid regex patterns (e.g. date prefixes) cause parse failure.

## Reproduction Steps
1. Save and run this new minimal script:

```python
import yaml

rendered = """receivers:
filelog:
multiline:
line_start_pattern: [0-9]{4}-[0-9]{2}-[0-9]{2}
omit_pattern: false
"""

print('--- Rendered YAML ---')
print(rendered)
print('--- Parse result ---')
yaml.safe_load(rendered)
```

2. Run:

```bash
python /tmp/gh-aw/agent/repro_filelog_otel_yaml.py
```

3. Observe deterministic parser failure.

## Expected vs Actual
**Expected:** A valid multiline regex value should render into valid YAML and be parseable by the agent config loader.

**Actual:** YAML parsing fails with:

```text
yaml.parser.ParserError: while parsing a block mapping
in "(unicode string)", line 4, column 7:
line_start_pattern: [0-9]{4}-[0- ...
^
expected (block end), but found '{'
in "(unicode string)", line 4, column 32:
line_start_pattern: [0-9]{4}-[0-9]{2}-[0-9]{2}
^
```

## Failing Test
```python
import yaml

rendered = """receivers:
filelog:
multiline:
line_start_pattern: [0-9]{4}-[0-9]{2}-[0-9]{2}
omit_pattern: false
"""

yaml.safe_load(rendered)
```

## Evidence
- Unquoted template rendering in `packages/filelog_otel/agent/input/input.yml.hbs`:
- `line_start_pattern: \{\{multiline_line_start_pattern}}` (line 39)
- `line_end_pattern: \{\{multiline_line_end_pattern}}` (line 44)
- User-exposed multiline regex vars in `packages/filelog_otel/manifest.yml`:
- `multiline_line_start_pattern` (line 195)
- `multiline_line_end_pattern` (line 201)
- No matching open duplicate found via issue search for `filelog_otel` + `multiline` + `line_start_pattern`.

---
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