Implement printf builtin command

.claude/skills/review-fix-loop/SKILL.md

@@ -143,6 +143,8 @@ Run the **address-pr-comments** skill:
 ```
 This reads all unresolved review comments, evaluates validity, implements fixes, commits, pushes, and replies/resolves threads.
 
+**Commit message prefix:** All commits created in this sub-step MUST be prefixed with the current loop iteration number, e.g. `[iter 3] Fix null check in parser`.
+
 Wait for completion before proceeding to 2C.
 
 ### Sub-step 2C — Fix CI failures
@@ -153,6 +155,8 @@ Run the **fix-ci-tests** skill:
 ```
 This checks for failing CI jobs, downloads logs, reproduces failures locally, fixes them, and pushes.
 
+**Commit message prefix:** All commits created in this sub-step MUST be prefixed with the current loop iteration number, e.g. `[iter 3] Fix flaky test timeout`.
+
 Wait for completion before proceeding to 2D.
 
 ---

SHELL_FEATURES.md

@@ -15,6 +15,7 @@ Blocked features are rejected before execution with exit code 2.
 - ✅ `grep [-EFGivclLnHhoqsxw] [-e PATTERN] [-m NUM] [-A NUM] [-B NUM] [-C NUM] PATTERN [FILE]...` — print lines that match patterns; uses RE2 regex engine (linear-time, no backtracking)
 - ✅ `head [-n N|-c N] [-q|-v] [-z] [FILE]...` — output the first part of files (default: first 10 lines)
 - ✅ `ls [-1aAdFhlpRrSt] [FILE]...` — list directory contents
+- ✅ `printf FORMAT [ARGUMENT]...` — format and print data to stdout; supports `%s`, `%b`, `%c`, `%d`, `%i`, `%o`, `%u`, `%x`, `%X`, `%e`, `%E`, `%f`, `%F`, `%g`, `%G`, `%%`; format reuse for excess arguments; `%n` rejected (security risk); `-v` rejected
 - ✅ `strings [-a] [-n MIN] [-t o|d|x] [-o] [-f] [-s SEP] [FILE]...` — print printable character sequences in files (default min length 4); offsets via `-t`/`-o`; filename prefix via `-f`; custom separator via `-s`
 - ✅ `tail [-n N|-c N] [-q|-v] [-z] [FILE]...` — output the last part of files (default: last 10 lines); supports `+N` offset mode; `-f`/`--follow` is rejected
 - ✅ `true` — return exit code 0