feat(coding-agent): added splice_block locators for atom edits
- Added `splice_block` atom edits with bracket-aware locators for block, body, head, and tail regions.
- Expanded `href` with optional `pre`/`post` args and updated anchor docs to describe bracketed wrapping behavior.
- Added delimiter and indent helpers to infer block delimiters, skip comments/strings, validate braces, and reindent inserts.
- Updated edit resolution to use block editing flow, enforce splice-only bracket locators, and reject mixed splice_block ops.
- Added unit and regression tests covering splice boundaries, unbalanced-brace cases, and locator validation errors.
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