docs: mandate Linux CLI tools for file editing over internal tools

Added requirement for AI agents to use standard Linux command-line
tools (sed, awk, grep, patch, cut, etc.) instead of internal text
editing functions.

Rationale:
- Internal editing tools fail frequently with whitespace/encoding issues
- Command-line tools are deterministic and well-tested
- Better error messages and easier verification workflow

Changes:
- AGENTS.md: Added "File Editing Requirements" section with tool usage
- SDLC.md: Added "File Editing Standards" section with patterns

Reference: User feedback that internal editing tools "fail way too much"

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AGENTS.md

@@ -434,3 +434,63 @@ Assisted-by: <AI-Model> via Crush <crush@charm.land>
 
 **Last Updated**: 2026-02-19
 **SDLC Enforcement**: Pre-commit hook + mandatory workflow documentation
+
+---
+
+## File Editing Requirements
+
+### Use Linux Command-Line Tools (MANDATORY)
+
+**When editing files, prefer Linux command-line tools over internal editing functions.**
+
+**Preferred Tools:**
+- `sed` - Stream editor for text transformations
+- `awk` - Pattern scanning and processing
+- `grep` - Search and filter text
+- `patch` - Apply diff files
+- `cut` - Remove sections from lines
+- `tr` - Translate/delete characters
+- `head`/`tail` - Output first/last lines
+- `sort`/`uniq` - Sort and deduplicate
+- `xargs` - Build command lines from input
+
+**When to Use Each:**
+```bash
+# Replace text in file
+sed -i 's/old/new/g' file.txt
+
+# Replace on specific line
+sed -i '42s/old/new/' file.txt
+
+# Append after line matching pattern
+sed -i '/pattern/a\new line' file.txt
+
+# Delete lines matching pattern
+sed -i '/pattern/d' file.txt
+
+# Extract specific column
+awk '{print $2}' file.txt
+
+# Process based on condition
+awk '/pattern/ {print $1, $3}' file.txt
+
+# Search and replace with regex
+sed -i -E 's/pattern/replacement/g' file.txt
+
+# Apply a patch
+patch -p1 < changes.diff
+```
+
+**Why This Matters:**
+- Internal editing tools fail frequently with whitespace/encoding issues
+- Command-line tools are deterministic and well-tested
+- Easier to verify changes before applying
+- Better error messages when something goes wrong
+- Can preview changes with `sed 's/old/new/g' file` (no -i) first
+
+**Workflow:**
+1. Read file first: `cat file.txt` or `head -n 50 file.txt`
+2. Preview change: `sed 's/old/new/g' file.txt` (no -i)
+3. Apply change: `sed -i 's/old/new/g' file.txt`
+4. Verify: `git diff file.txt`
+