feat(setup): multi-shell integration with completions and custom names

[reckerp]

What

Polishes gx setup so shell integration covers more shells, ships completion, supports custom wrapper names, and handles branch-name globs safely. Implements section 6 (Shell Integration Polish) of the workspace improvement plan.

New CLI surface

gx setup                      # auto-detects shell from $SHELL
gx setup --shell zsh|bash|fish
gx setup --completions zsh    # emit only the static completion script
gx setup --name gx-dev --command /path/to/gx

Changes

• Multi-shell wrappers: replaced the single POSIX wrapper with shell-specific integration for zsh, bash, and fish. Shell is detected from $SHELL, with --shell as an override.
• zsh noglob: the zsh wrapper aliases through noglob so branch-name arguments containing glob characters (gx workspace remove feat/*, gx checkout users/[id]) reach gx literally instead of being expanded by the shell.
• Completion: static command/flag completion is generated via clap_complete, then layered with dynamic helpers that provide live candidates for workspace names, branch names, remote branch names, and stash refs.
• Custom command names: --name/--command emit integration for a custom wrapper name pointing at a specific binary, useful for developing gx locally while keeping the installed release available as gx.
• __complete backend: a hidden gx __complete <kind> invocation backs the dynamic completion helpers. It is intercepted in main before clap parses rather than being a clap subcommand, because clap_complete emits even hidden subcommands into the static completion script and would otherwise surface it as a visible gx <TAB> candidate.
• git helper: added get_remote_branches for remote-tracking branch completion (skips symbolic refs like origin/HEAD).

Conventions honored

• stdout stays reserved for the eval'd script and the __complete candidate list; all human-facing notices go to stderr.
• Existing alias generation behavior is kept stable.

Why

gx setup only supported a single POSIX/zsh-style wrapper with no completion and no way to point at an alternate binary. This makes the integration usable across zsh, bash, and fish, adds completion that understands gx's own workspace/branch/stash state, and removes a class of breakage where the shell expanded branch-name globs before gx saw them.

Testing

• cargo build passes with no warnings.
• cargo test passes (145 tests). 15 new unit tests cover shell detection, per-shell wrapper contents, noglob presence in zsh, alias rendering and sorting, custom-name substitution, dynamic completion wiring for all four kinds across all three shells, and that the internal __complete helper never leaks into static completion output.