Implement uniq builtin command

[AlexandreYang]

What does this PR do?

Implements the POSIX uniq command as a builtin in the safe shell interpreter. The command filters adjacent matching lines from input, with support for counting, duplicate/unique filtering, case-insensitive comparison, field/character skipping, and group display modes.

Motivation

Extends the shell's builtin command set with uniq, a commonly used text-processing utility needed for deduplication workflows in shell scripts.

Testing

• Go unit tests (interp/builtins/uniq/uniq_test.go): 60+ tests covering all flags, edge cases, error paths, context cancellation, CRLF, binary data, stdin, pipes
• GNU compat tests (interp/builtin_uniq_gnu_compat_test.go): 24 tests verifying byte-for-byte output equivalence with GNU coreutils
• Pentest tests (interp/builtin_uniq_pentest_test.go): 26 tests covering integer edge cases, path injection, flag injection, large files, timeout, binary content
• YAML scenario tests (tests/scenarios/cmd/uniq/): 27 scenarios across basic, count, repeated, unique, skip, check_chars, ignore_case, zero_terminated, all_repeated, group, stdin, errors, and hardening categories
• All tests pass: go test ./interp/builtins/uniq/ ./interp/ ./tests/ ✅

Checklist

• Tests added/updated
• Documentation updated (if applicable)

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Summary of changes:

• interp/builtins/uniq/uniq.go — Full implementation with flags: -c, -d, -D, -f, -i, -s, -u, -w, -z, --group, -h/--help. Streaming processing (O(1) memory for default and --group modes), capped buffering for --all-repeated. All writes error-checked, context cancellation respected in all loops.
• interp/builtins/uniq/uniq_test.go — Comprehensive unit tests
• interp/builtin_uniq_gnu_compat_test.go — GNU output equivalence tests
• interp/builtin_uniq_pentest_test.go — Security/hardening tests
• interp/runner_exec.go — Register uniq package import
• tests/import_allowlist_test.go — Add safe symbols: bufio.Scanner, bufio.SplitFunc, io.WriteString, io.Writer, strconv.Itoa, strings.EqualFold
• tests/scenarios/cmd/uniq/ — 27 YAML scenario files
• SHELL_COMMANDS.md — Added uniq to command reference table

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Bits Dev status: ✅ Done

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