bug: prevent tmux project listing from failing on empty roots

[ALX99]

Bug

.local/bin/tmux-go-session --list fails when a configured project root exists but contains no child directories. Bash leaves the unmatched "$d"/*/ pattern literal, and the script later tries to derive a tmux session name from the literal * basename.

Severity and impact

This is a reliability bug in the tmux project picker. An empty project root is a normal state on a new machine, after moving projects, or when one of several configured roots is temporarily empty. Instead of continuing with existing tmux sessions and the built-in ~/dotfiles / ~/.claude entries, the picker exits non-zero and cannot open.

Evidence

Relevant code on master:

• .local/bin/tmux-go-session, emit_picker_rows() loops over for sub in "$d"/*/ without enabling nullglob or checking that each expanded value is a directory.
• When $d is empty, Bash supplies the literal value $d/*/ to the loop.
• After removing the trailing slash, ${sub##*/} is *.
• Session-name sanitization converts * to _, strips the leading/trailing underscore, and produces an empty name.
• The existing guard then prints Cannot derive a tmux session name for project: <root>/* and returns 1.

The added regression test creates an existing but empty project root, stubs tmux list-sessions, runs tmux-go-session --list, and verifies that listing succeeds without leaking the unmatched glob while preserving the built-in picker entries.

This is a real defect rather than a hypothetical edge case because an existing empty directory deterministically triggers Bash's default unmatched-glob behavior.

Reproduce

Setup:

git checkout master
tmp=$(mktemp -d)
mkdir -p "$tmp/bin" "$tmp/home/dotfiles" "$tmp/home/.claude" "$tmp/projects"
printf '#!/usr/bin/env bash\nexit 1\n' > "$tmp/bin/tmux"
chmod +x "$tmp/bin/tmux"

Triggering condition: PROJ_DIR names an existing directory with no child project directories.

Exact command:

HOME="$tmp/home" PROJ_DIR="$tmp/projects" PATH="$tmp/bin:$PATH" \
  .local/bin/tmux-go-session --list

Actual result on master:

Cannot derive a tmux session name for project: <tmp>/projects/*

The command exits non-zero.

Expected result:

The command exits successfully, ignores the unmatched project glob, and continues listing valid session/project entries.

Reviewer regression command:

bash tests/tmux-go-session-empty-project-root.test.sh

Root cause

The project enumeration loop assumes "$d"/*/ always expands to at least one real directory. Bash does not remove unmatched globs by default, so an empty root produces a literal glob string that is treated as a project path.

Fix

Add a directory guard at the start of the inner loop:

[[ -d $sub ]] || continue

This skips the unmatched literal and also defensively ignores any non-directory value without changing enumeration of real project directories.

Validation

Validation performed:

• Checked existing open and closed pull requests for the same tmux-go-session empty-root/glob defect; no duplicate was found.
• Checked open issues for the same defect; no duplicate was found.
• Traced the baseline Bash expansion and sanitization path deterministically: empty root -> literal <root>/*/ -> basename * -> sanitized empty name -> existing error return.
• Added tests/tmux-go-session-empty-project-root.test.sh, which stubs tmux, creates an empty project root, and asserts successful listing with no leaked glob.
• Compared master...bug/tmux-go-session-empty-project-root: 2 commits ahead, 0 behind; only .local/bin/tmux-go-session and the focused regression test changed.

Commands not run in this connector-only runtime:

bash tests/tmux-go-session-empty-project-root.test.sh
bash -n .local/bin/tmux-go-session tests/tmux-go-session-empty-project-root.test.sh
shellcheck .local/bin/tmux-go-session tests/tmux-go-session-empty-project-root.test.sh

No command output is claimed for those checks.

Scope

Intentionally not changed:

• project-root parsing through PROJ_DIR,
• session sorting or filtering,
• project-name collision handling,
• built-in ~/dotfiles and ~/.claude entries,
• picker behavior, previews, session creation, or session killing,
• global shell glob settings,
• unrelated scripts or configuration.