fix: complete Git share snapshot flags

[steipete]

Summary

• expose publish --tag in generated Bash and Zsh completions
• expose update --ref in generated Bash and Zsh completions
• add regression coverage and changelog entry

Found during post-merge verification of #64.

Proof

• go test -count=1 ./internal/cli -run 'TestCompletion(Bash|Zsh)Output'
• go test -count=1 ./...
• go test -race -count=1 ./...
• go vet ./...
• go run golang.org/x/tools/cmd/deadcode@v0.45.0 -test ./...
• go mod verify
• go run golang.org/x/vuln/cmd/govulncheck@v1.3.0 ./...
• autoreview clean

[clawsweeper]

ClawSweeper status: review started.

I am starting a fresh review of this pull request: fix: complete Git share snapshot flags This is item 1/1 in the current shard. Shard 0/1.

This placeholder means the worker is alive and reading the current context. I will edit this same comment with the actual review when the claws are done clicking.

Crustacean status: shell secured, claws on keyboard, evidence pebbles being sorted.

[steipete]

Landed as d1421a90f70bbba3cf31f60773c7b3deb8e1025a.

• Added Bash/Zsh completion coverage for publish --tag and update --ref, with regression tests and changelog entry.
• Focused, full, race, vet, deadcode, module verification, and vulnerability gates passed locally.
• autoreview --mode local returned clean with no accepted/actionable findings.
• Exact merged-head CI, Docker, CodeQL, and secret scanning are green.