Add bash and zsh completions

[leogdion]

Adds bash and zsh completions for git trees.

The dispatch naming (most likely thing to review wrong)

The tool installs on PATH as git-trees and is invoked as git trees. Two
different shells resolve that subcommand two different ways, and both names are
load-bearing:

• bash — bash-completion's git driver does
  local completion_func="_git_${command//-/_}", so git trees dispatches to a
  function named _git_trees (underscores, not dashes). Verified against
  /Library/Developer/CommandLineTools/usr/share/git-core/git-completion.bash:3830.
• zsh — _git does _call_function ret _git-$words[1], so git trees
  dispatches to a file named _git-trees (dashes) found on fpath.

Renaming either one silently disables completion with no error, so both files
carry a comment explaining why the name cannot change, and the README says so
too.

Each file also wires up direct invocation of the standalone git-trees binary
(complete -F in bash, the #compdef git-trees tag in zsh), so completion
works whether the user types git trees or git-trees.

What is completed

Subcommands at position 1 (init root add track list ls rm clean sync prune help), then each subcommand's own flags:

command	flags
init	--host --dir
root	--agents
add	--print-path --no-push
track	--no-push
list / ls	--json
rm	--apply
clean	--merged --gone --apply
sync	--pull --ff-only --rebase
prune	--dry-run

rm completes branch and worktree names derived from git itself
(git for-each-ref refs/heads plus git worktree list --porcelain), never a
hardcoded list. Worktree directory names are branch names slugged /→-, so
the two sets overlap heavily and both files dedupe. The bare container root
(trees-bare.git) is filtered out — git lists it as a worktree but it is not a
removable target.

Every git call is silenced and short-circuited: completing outside a repository
returns an empty list rather than erroring or hanging in the user's prompt.
There is a smoke assertion for exactly that.

sync / prune anticipate sibling PRs

sync [worktree] [--pull] [--ff-only|--rebase] and prune [--dry-run] are
being added concurrently in #50 and #55. They are included here so the
completions are correct for the release as a whole. This PR merges after those
two, so it may need a rebase on release-1.0.3 first — the completions
themselves need no change either way, since they do not read anything out of
git-trees at runtime.

Verified

bash -n on all four bash files, shellcheck -s bash on all four,
zsh -n completions/_git-trees, and tests/smoke.sh ./git-trees all pass
clean.

Beyond the linters, the bash completion was exercised for real: sourcing
bash-completion's git script plus this file into a non-interactive bash,
confirming _git_trees is defined, and driving COMP_WORDS/COMP_CWORD
through it. Every row of the table above was checked, plus the empty-and-quiet
behavior outside a repo. Those probes are now part of tests/smoke.sh so they
run in CI on both runners.

For zsh, zsh -n passes and a compinit load test confirms the dispatch wiring
resolves: with completions/ on fpath, _comps[git-trees] resolves to
_git-trees and the function autoloads, defining its helpers and running its
_arguments specs. Full interactive match capture was not possible in this
environment — zsh/zpty here does not deliver keystrokes to ZLE (a trivial
control-key-bound widget never fires), and _arguments refuses to run outside a
real completion context. That limitation is the sandbox's, not the file's, and
is called out rather than glossed over.

Why the zsh file is excluded from CI linting

completions/git-trees.bash is added to both the bash -n and shellcheck
steps. completions/_git-trees is added to neither, deliberately: #compdef,
_arguments, ${(f)...} and friends are zsh syntax that is not valid bash, and
including it would fail both steps. It is covered by zsh -n locally and by the
smoke test asserting install.sh places it byte-identical to the source.

Install and docs

install.sh copies both files to ~/.config/git-trees/completions/ following
the existing AGENTS.md.template shape — a secondary artifact with a
no-overwrite guard — then prints where each landed and the one line needed to
activate it. A user who edits an installed copy keeps it across reinstalls;
there are smoke assertions for both the install and the guard. Every existing
install.sh behavior is preserved, and the edit is additive and tightly scoped so
it should merge cleanly with the curl | bash bootstrap work in #54.

README.md gains a ### Shell completions subsection under ## Install
covering both shells, including what to do manually if you installed via the
curl path rather than install.sh.

Closes #51

🤖 Generated with Claude Code

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Release coordination (v1.0.3)

One of five PRs into release-1.0.3 (#56 prune, #58 sync, #60 completions, #59 curl install, #57 Homebrew). All five are green on CI (smoke on Linux + macOS).

Suggested merge order: #56 → #58 → #60 → #59 → #57. This PR merges cleanly against every other branch.

⚠️ Verification gap: zsh is structurally verified only

The bash completion is fully exercised — sourced against the system git-completion.bash, with real COMPREPLY output confirmed for every subcommand and flag set. The zsh side is not. zsh -n passes and compinit resolves the dispatch (_comps[git-trees] → _git-trees), but no actual tab-completion was ever driven through it: that needs real ZLE, and zsh/zpty in the sandbox would not deliver keystrokes to it. Please tab-test git trees <TAB> in a real zsh before merging.

Also note this PR anticipates #58 and #56: sync and prune are already in both completion files with their flags. If either of those PRs changes a flag name before merging, these lists need to follow.

Note: CodeRabbit skipped all five — "reviews are disabled for this base branch." These have not had automated review; that would come when release-1.0.3 merges to main.

The CHANGELOG is deliberately excluded from every PR — it needs the merged PR URLs, so it lands as one commit on release-1.0.3 before tagging v1.0.3.

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