feat: cora index — Symbol index & SQLite persistence #264

Description

Build a persistent symbol index for projects using existing AST cross-file dependency extraction (v0.4, #114).

When cora index is run, it should:

• Parse source files using existing AST pipeline
• Extract function/class/method/variable/type declarations
• Build a symbol table with file location, kind, and metadata
• Store in local SQLite database (.cora/index.db with FTS5 for full-text search)
• Support incremental re-index (only changed files)

Why

Foundation for code intelligence. Without a persistent index, there's nothing to query. This is the storage layer that cora explore, cora callers, cora impact, and cora affected will all build on.

Scope

• Leverage existing src/ast.rs cross-file dependency extraction
• New src/index.rs module for index building and storage
• SQLite schema with symbols table + FTS5 virtual table
• File fingerprinting (mtime + content hash) for incremental sync

Depends On

• feat: Deterministic context chain — AST-based cross-file dependency extraction #114 AST-based cross-file dependency extraction (done)
• Feature: Review history and learning via Uteke memory integration #232 Uteke memory integration (done)

Strategy Ref

Code intelligence strategy #1 from competitive analysis.