Inspired by openai/codex's apply_patch implementation, this changes the edit_files search-and-replace to use a cascading match strategy when the exact search string isn't found:

1. Exact substring match (byte-for-byte) — existing behavior, unchanged
2. Line-by-line match ignoring trailing whitespace — handles trailing spaces/tabs the LLM omits
3. Line-by-line match ignoring all leading/trailing whitespace — handles tabs-vs-spaces and wrong indentation depth

Problem

When the chat agent uses edit_files, it generates a search string that must match the file content exactly. LLMs frequently get whitespace wrong:

• Emitting spaces when the file uses tabs (or vice versa)
• Getting the indentation depth wrong by one or more levels
• Omitting trailing whitespace that exists in the file

When this happens, the edit silently does nothing, and the agent falls into a retry loop using cat -A to diagnose the exact whitespace characters.

Solution

Adopted the same cascading fuzzy match strategy that openai/codex uses in seek_sequence.rs:

• Pass 1: exact match (existing behavior)
• Pass 2: TrimRight each line before comparing (trailing whitespace tolerance)
• Pass 3: TrimSpace each line before comparing (full indentation tolerance)

When a fuzzy match is found, the matched lines in the original file are replaced with the replacement text. This preserves surrounding content exactly.

Changes

• agent/agentfiles/files.go: Replaced icholy/replace streaming transformer with in-memory fuzzyReplace + helper functions (seekLines, spliceLines)
• agent/agentfiles/files_test.go: Added 6 new test cases covering trailing whitespace, tabs-vs-spaces, different indent depths, exact match preference, no-match behavior, and mixed whitespace multiline edits
• Removed icholy/replace dependency from go.mod/go.sum