feat(agent): add fuzzy whitespace matching to edit_files tool (#22446)

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`](https://github.com/openai/codex/blob/main/codex-rs/apply-patch/src/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

---------

Co-authored-by: Kyle Carberry <kylecarbs@users.noreply.github.com>

go.sum

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