Add an optional ignore_case flag to the grep tool #36

Problem

The built-in grep tool searches case-sensitively by default. When a user asks the agent to find an error message or a term whose capitalization is unknown, the agent must know to add Go's inline (?i) regex flag or try several searches. That workaround already works, but case-insensitive search is not discoverable in the tool schema.

Proposed improvement

Add an optional boolean ignore_case argument to grep, described in the tool schema. Keep the existing behavior when the argument is omitted or false, including support for inline regex flags.

For example, given a file containing Error: connection failed, ERROR: timeout, and error: retrying, this call should return all three matching lines:

{"pattern":"error", "path":".", "include":"*.log", "ignore_case":true}

This is a small convenience feature for searches in logs, documentation, and unfamiliar repositories.

Acceptance criteria

• The grep tool schema exposes ignore_case as an optional boolean and explains its default.
• With ignore_case: true, a pattern such as error matches error, Error, and ERROR in both single-file and recursive searches.
• Omitted, false, or null values preserve today's regex behavior, including explicit inline flags such as (?i); invalid non-boolean values produce an actionable tool error, consistent with replace_all.
• Case folding applies to matching file contents only. The include glob, returned paths, line numbers, and original line text retain their current behavior.
• Invalid regexes still report an error; existing binary/large-file exclusions and match limits remain intact.
• Tests cover the default and enabled modes, interaction with include, argument validation through the executor, and the optional boolean schema property. go test ./... passes.
• Add a brief README note with an example of case-insensitive search.

Implementation pointers

The change should fit within internal/tools/grep.go, the argument/schema/dispatch handling in internal/tools/tools.go, the existing grep tests in internal/tools/tools_test.go, and the README. Reuse the existing boolArg convention. No new dependency or CLI flag is needed.