fix(hermes): preserve YAML list-at-same-indent style on install (#456) (#461)

Hermes Agent writes ~/.hermes/config.yaml with PyYAML's default block
style, which puts list items at the SAME indent as the parent key:

    platform_toolsets:
      cli:
      - hermes-cli      # indent 2, same as `cli:`
      - browser

The previous line-based YAML patcher used `^  \S` to find the end of
the `cli:` block, which mistook that first `  - hermes-cli` line for
the next sibling key, truncated the block, and spliced
`    - mcp-codegraph` at indent 4 BEFORE the existing items. The
result was unparseable YAML: every subsequent item (`- browser`,
`- clarify`, …) and every sibling platform (`telegram:`, `discord:`)
appeared at the `platform_toolsets:` level. Hermes silently fell back
to the default config, dropping every user override.

The new `listChildBlock` helper recognizes `  - ` as a list-item
continuation (not a sibling key), finds the real end of the block at
the next sibling mapping key, and detects the existing item indent so
the new entry matches it. Two regression tests cover the PyYAML-default
style; the existing 4-space-nested test still passes.

End-to-end verified against a real `hermes-agent` install on the exact
bug-triggering config: `hermes mcp list` shows codegraph as enabled,
`hermes tools --summary` lists both `mcp-codegraph` and `codegraph` in
the CLI toolset, and `hermes mcp test codegraph` connects in 264ms and
discovers all 10 codegraph tools. Re-running `codegraph install`
reports `Unchanged` and the file still has exactly one entry. Closes #456.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>