feat: .ksh extension + shebang-based language detection for extension-less scripts (#235, #237)

[azizur100389]

Summary

Two parser improvements that expand file coverage to extension-less Unix scripts and Korn shell files. +288 lines, 18 new tests.

Closes #235, closes #237.

Supersedes individual PRs #236 and #238 (which are being closed in favor of this bundled PR).

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Feature 1: .ksh extension → bash parser (#235)

Register .ksh with tree-sitter-bash alongside .sh / .bash / .zsh. Korn shell is close enough to bash syntactically that tree-sitter-bash handles it correctly.

Context: @tirth8205 explicitly invited this in the close comment on PR #230:

The .ksh extension in particular looks worth adding — I didn't include it in #227.

Tests: test_detects_language extended with .ksh; test_ksh_extension_parses_as_bash — end-to-end parse that asserts identical function set and edge counts to .sh.

Feature 2: Shebang-based language detection (#237)

detect_language() was extension-only — any file with no extension was silently skipped. This misses git hooks, CI scripts, bin/ entry points, and installers.

New SHEBANG_INTERPRETER_TO_LANGUAGE table maps interpreter basenames to already-registered languages:

Interpreter	Language
bash, sh, zsh, ksh, dash, ash	bash
python, python2, python3, pypy, pypy3	python
node, nodejs	javascript
ruby, perl, lua, Rscript, php	(respective)

New _detect_language_from_shebang(path) reads first 256 bytes. Handles:

• Direct form: #!/bin/bash
• env indirection: #!/usr/bin/env bash
• env -S flag: #!/usr/bin/env -S node --experimental-vm-modules
• Trailing flags: #!/bin/bash -e
• CRLF line endings, binary content, strict UTF-8 decoding

detect_language() falls back to shebang probe only for files with suffix == "". Files with a known extension are never re-read.

Tests (16 new): every mapped interpreter, env -S flag, trailing flags, missing shebang, empty file, binary content, unknown interpreter, extension-override guard, and end-to-end parse_file() producing function nodes.

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Files changed (3 files, +288 / -1)

• code_review_graph/parser.py — .ksh mapping + SHEBANG_INTERPRETER_TO_LANGUAGE table + _detect_language_from_shebang() + detect_language() fallback
• tests/test_multilang.py — .ksh detection + end-to-end ksh parsing test
• tests/test_parser.py — 16 shebang detection tests

Test results

• 17/17 targeted tests pass (1 ksh + 16 shebang)
• ruff check on all changed files: clean (1 pre-existing F841 in an unrelated test)