fix(builtins): unescape \/ in sed replacement strings (#1028)

## Summary
- Fix `sed` builtin to properly unescape `\/` in replacement strings,
producing literal `/`
- Also handles `\\` → `\`, `\n` → newline, `\t` → tab in correct order
using placeholder approach
- Adds spec tests for slash unescaping, back-reference groups with
slashes, and escaped backslash

## Test plan
- [x] `sed_unescape_slash_in_replacement` — `\/` produces `/`
- [x] `sed_regex_groups_with_slash` — back-references with `\/` in date
reformatting
- [x] `sed_escaped_backslash_in_replacement` — `\\` produces `\`
- [x] All existing sed spec tests still pass
- [x] Smoke tested via CLI: `echo "abc" | sed 's/b/\//'` → `a/c`

Closes #959

Author: chaliy

crates/bashkit/src/builtins/sed.rs

@@ -515,8 +515,13 @@ fn parse_sed_command(s: &str, extended_regex: bool) -> Result<(Option<Address>,
                 .replace_all(&replacement, r"$${$1}")
                 .to_string();
 
-            // Convert \n → newline, \t → tab in replacement
-            let replacement = replacement.replace("\\n", "\n").replace("\\t", "\t");
+            // Convert \n → newline, \t → tab, \/ → /, \\ → \ in replacement
+            let replacement = replacement
+                .replace("\\\\", "\x01") // Temporarily escape literal \\
+                .replace("\\n", "\n")
+                .replace("\\t", "\t")
+                .replace("\\/", "/")
+                .replace("\x01", "\\"); // Restore literal backslash
 
             // Parse nth occurrence from flags (e.g., "2" in s/a/b/2)
             let nth = flags

crates/bashkit/tests/spec_cases/sed/sed.test.sh

@@ -552,3 +552,24 @@ BAR
 BAR
 baz
 ### end
+
+### sed_unescape_slash_in_replacement
+# \/ in replacement should produce literal /
+echo "abc" | sed 's/b/\//'
+### expect
+a/c
+### end
+
+### sed_regex_groups_with_slash
+# back-references with \/ in replacement
+echo "2026-01-15" | sed 's/\([0-9]*\)-\([0-9]*\)-\([0-9]*\)/\3\/\2\/\1/'
+### expect
+15/01/2026
+### end
+
+### sed_escaped_backslash_in_replacement
+# \\ in replacement should produce literal backslash
+echo "abc" | sed 's/b/\\/'
+### expect
+a\c
+### end

supply-chain/config.toml

@@ -579,7 +579,7 @@ version = "1.2.1"
 criteria = "safe-to-deploy"
 
 [[exemptions.indexmap]]
-version = "2.13.0"
+version = "2.13.1"
 criteria = "safe-to-deploy"
 
 [[exemptions.insta]]