feat: `cora explore` — Structural + semantic code search

[ajianaz]

Description

Query the symbol index (from cora index) with structural and semantic search.

cora explore "how does auth work"        # semantic search across index
cora explore "AuthService.validate"      # exact symbol lookup
cora explore --file src/auth.rs           # explore single file context

Should return:

• Relevant symbols with source context (name, file, line, signature)
• Call paths showing how symbols connect
• Blast radius — what depends on the result

Why

Primary query interface for code intelligence. The command agents and developers use 90% of the time. Inspired by CodeGraph's codegraph explore but with Uteke semantic recall layered on top — a combination no competitor offers.

Scope

• Query parser (natural language → structured query)
• SQLite FTS5 full-text search on symbol names/signatures
• Uteke semantic recall for context (optional, via --memory flag)
• Terminal table + JSON output formatting

Depends On

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

Strategy Ref

Code intelligence strategy #1 from competitive analysis.