feat(install-lang): seed a neutral CLAUDE.md when a bundle ships none

install-lang only seeded CLAUDE.md if the chosen bundle shipped one. elisp and go do, python and typescript don't. A project installing a template-less bundle got no CLAUDE.md, and a multi-bundle install inherited whichever bundle shipped one. A bash project that installed elisp and python ended up headed "Elisp project," worse than no header.

I added a language-neutral default (languages/default-CLAUDE.md) that names no language, so single-language, multi-bundle, and wrong-bundle installs all get an accurate "fill this in" header instead of a false one. Per-bundle templates still win where present. The seed-on-first-install, no-overwrite logic is unchanged.

I hardened the Makefile LANGUAGES glob to directories only so the new template file doesn't show up as a selectable language. lint covers the default. The install-lang tests cover the fallback, the bundle-wins branch, and no-overwrite.

A bash bundle is still the real gap for shell-heavy projects, filed as a backlog task.

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+# CLAUDE.md
+
+## Project
+
+Describe this project: what it is, its layout, and its conventions. This
+default was seeded by `install-lang` because the installed bundle ships no
+language-specific CLAUDE.md — it deliberately names no language, so replace
+this section with an accurate description rather than inheriting a wrong one.
+
+**Typical layout (edit to match):**
+- entry points — the file(s) that run first
+- source directories — where the real code lives
+- tests — beside the code, or under a `tests/` tree
+
+## Build & Test Commands
+
+If the project has a Makefile, document its targets here. A common shape:
+
+```bash
+make test       # run the test suite
+make lint       # run the linter / formatter check
+make build      # build the project
+```
+
+Otherwise, document the direct commands a contributor runs to test and build.
+
+## Language Rules
+
+Shared rules live in `.claude/rules/` (installed from `claude-rules/`):
+- `commits.md` — author identity, no AI attribution, message format
+- `testing.md` — TDD discipline and test-quality standards
+- `verification.md` — verify-before-claim-done discipline
+
+If a language bundle was installed, its own rule files (code style, testing
+conventions) sit alongside these in `.claude/rules/`.
+
+## Git Workflow
+
+Commit conventions: see `.claude/rules/commits.md`.
+
+If a `githooks/` pre-commit hook was installed, activate it on a fresh clone
+with `git config core.hooksPath githooks`.
+
+## Problem-Solving Approach
+
+Investigate before fixing. When diagnosing a bug:
+1. Read the relevant code and trace what actually happens
+2. Identify the root cause, not a surface symptom
+3. Write a failing test that captures the correct behavior
+4. Fix, then re-run tests
+
+## Testing Discipline
+
+TDD is the default: write a failing test before any implementation. If you
+can't write the test, you don't yet understand the change. Details in
+`.claude/rules/testing.md`.
+
+## What Not to Do
+
+- Don't add features beyond what was asked
+- Don't refactor surrounding code when fixing a bug
+- Don't add comments to code you didn't change
+- Don't create abstractions for one-time operations
+- Don't commit credentials or API keys