[tui] The LSP scenarios fail on their second run: a read-only module cache in the throwaway HOME #115

Context

The gated LSP scenarios cannot be run twice in a row. make test-tui-lsp passes on a fresh
/tmp and the second run fails:

== tui/testdata/scenarios/lsp-diagnostics.scenario
ok: 14 steps
== tui/testdata/scenarios/lsp-diagnostics.scenario
    Error: LLM call failed: api call: Post "http://127.0.0.1:18511/v1/chat/completions":
    dial tcp 127.0.0.1:18511: connect: connection refused

The connection error says nothing about LSP. Each scenario runs the binary with
HOME=/tmp/tinyscen-home-lsp and opens with

rm -rf /tmp/tinyscen-home-lsp && mkdir -p ... && python3 .../openai_stub.py 18511 read & sleep 1; ./bin/tinycode ...

gopls resolves the workspace through that redirected HOME, so it fills $HOME/go/pkg/mod with
the Go module cache. Those directories are read-only (0555), rm -rf needs write permission on
them, so the wipe exits 1, the && chain stops before the stub is started, and the TUI
reports a connection error against a server nobody launched.

Measured on this machine (macOS, gopls 0.23.0, Go 1.27.1):

CI never sees it: the runner's /tmp starts empty and each job runs the scenarios once. The
failure mode is the expensive part — a wipe that fails is invisible, and what surfaces is an
unrelated connection error.

What to build

• tui/testdata/scenarios/lsp-*.scenario: drop the write permission before the wipe, and keep
  the wipe outside the && chain that starts the stub —
  chmod -R u+w <home> 2>/dev/null; rm -rf <home>; mkdir -p ... && printf ... && python3 ... & sleep 1; ...
  so a wipe that somehow still fails cannot stop the stub from starting.
• make test-tui-lsp runs every lsp-*.scenario: it names lsp-diagnostics.scenario
  explicitly today, so a second LSP scenario would be skipped in silence.

Acceptance

• With gopls on PATH, make test-tui-lsp run twice in a row exits 0 both times, with
  ok: <n> steps for every lsp-*.scenario.
• Capture recipe if it returns: make test-tui-lsp > /tmp/lsp.txt 2>&1, then read /tmp/lsp.txt.
  connection refused means the stub never started (this issue); any other failure belongs to
  the assertion that failed, not to the wipe.
• make test-tui-scenarios still skips every lsp-* file with the "run make test-tui-lsp" line.