feat: add symbols tool — file symbol tree with line numbers (#546)

* feat: add symbols tool — file symbol tree with line numbers

Add a new `symbols` command that lists all symbols (functions, structs,
classes, constants, enums, traits, impl blocks, type aliases, etc.) in
files as a table of contents with line numbers and nesting.

Rust implementation uses tree-sitter AST parsing with a new
`is_symbol_node()` trait method (broader than `is_acceptable_parent`)
and recursive descent for nested symbols (methods inside impl/class).

Full pipeline: CLI subcommand → Node.js wrapper → Vercel AI SDK tool →
agent registration (schema, system prompt, predefined prompts).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: address symbols tool review feedback

- Refactor processor.rs extract_all_symbols_from_file to delegate to
  symbols::extract_symbols, eliminating duplicate AST traversal logic
- Fix Vercel tool to return full result array instead of only first element
- Add escapeString for file paths in symbols.js matching extract.js pattern

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: address Visor review feedback on symbols tool

- Extract depth limit to named constant MAX_SYMBOL_DEPTH with docs
- Pre-allocate Vec capacity in collect_symbols and format_symbols_text
- Simplify Vercel tool return: always result[0] (schema takes single file)
- Return consistent JSON for empty results instead of plain string
- Improve symbolsSchema description to mention hierarchical nesting
- Include stdout preview in JSON parse error messages
- Clarify processor.rs legacy wrapper documentation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>