fix: exact-name symbol lookup silently fails on snake_case names

[saldestechnology]

Problem

query find, query callers, query deps, query impact, explain, and source return "not found" for any symbol whose name contains an underscore — i.e. most snake_case identifiers in Rust/Go/Python/C codebases.

$ ctx query find get            # ✅ found
$ ctx query find run_sql        # ⛔ "No symbols found"
$ ctx query find discover_files # ⛔ "No symbols found"
$ ctx query impact run_audit    # ⚠️ "No impact detected"  ← silently wrong

The symbols are indexed — ctx sql, search, and semantic all find them. Only the LIKE-based exact-name path fails.

Root cause

find_symbols_filtered (src/db/schema.rs) escapes LIKE metacharacters (%, _, \) with a backslash (escape_like_pattern, and glob_to_like_pattern for the file filter), but none of the LIKE clauses declared ESCAPE '\'. In SQLite the backslash is not the escape character unless declared, so the escaped \_ was matched literally as "backslash + any char" — which never matches a name like run_sql. Substrings without underscores matched fine, which is why the bug hid in plain sight.

The impact variant is the most dangerous: it returns "No impact detected" rather than an error, which a caller (human or agent) reads as "safe to change."

Fix

Add ESCAPE '\' to all three affected LIKE clauses:

• name / qualified_name substring match
• the file_path glob filter
• the starts_with prefix match in the ORDER BY

Why the tests missed it

The existing find_symbols_filtered tests only used underscore-free names (new, main, targetfn). This PR adds test_find_symbols_with_underscore_name, which looks up a snake_case name by exact match and asserts the underscore is treated literally (does not wildcard-match runsql) and that exact match still ranks ahead of the longer prefix match.

Verification

• New regression test fails before the fix, passes after.
• Full suite green: 379 tests, 0 failures; cargo fmt --check and cargo clippy --all-targets -D warnings clean.
• End-to-end against a real index: query find run_sql, explain open_sql_sandbox, and query impact find_symbols_filtered all now resolve correctly.