[ybai08]

Fixes #558

With a namespace such as type Migration mg.Namespace, mage -autocomplete prints migration:create, migration:down, ... but neither the zsh nor the bash script completes them.

zsh: _describe reads each entry as name:description, so migration:create was offered as migration with the description create, and the other targets in the namespace were dropped. The script now escapes the colons (${(@)targets//:/\\:}) before calling _describe.

bash: : is in COMP_WORDBREAKS by default, so the colon splits the word. On bash 3.2 (macOS), mage migration:<TAB> turned into mage migration:migration:. On bash 4+, COMP_WORDS[COMP_CWORD] is just the text after the colon, so nothing matched. The script now takes the current word from COMP_LINE up to COMP_POINT. When : is a word break, it also removes the part up to the last : from each match, because bash only replaces the text after that point. This is the same approach bash-completion's __ltrim_colon_completions uses, without depending on bash-completion. fish and PowerShell are not affected and are unchanged.

Tests: TestBashCompletionNamespacedTargets sources the generated bash script with a stand-in mage that prints namespaced targets, sets COMP_LINE/COMP_POINT/COMP_WORDS the way bash does, and checks COMPREPLY. TestZshCompletionNamespacedTargets stubs _describe and checks that the target names reach it with their colons escaped. Both skip when the shell isn't installed, and both skip on Windows. Before the fix, 3 of the 6 bash cases and the zsh test fail.

What was run (macOS arm64, Go 1.27.1, zsh 5.9, /bin/bash 3.2.57):

• go test -v -vet=all -tags CI -race ./...: everything passes except TestBootstrap. That test fails the same way on master here, because GOPATH isn't set in the environment (stat bin/mage).
• go vet -tags CI ./... and gofmt -l .: clean.
• golangci-lint run ./...: no findings in the changed files. CI pins v2.11.2, but that version can't load Go 1.27 export data, so v2.15.0 was used. Its only findings are 156 exhaustruct_v5 hits that are identical on master; that linter is newer than the repo's config.
• Real interactive shells, driven through a pty with a magefile containing a namespace: zsh (via zpty) now offers migration:create migration:down migration:up for mage <TAB> and mage migration:<TAB>, and migration:down for mage migration:d<TAB>. bash 3.2 completes mage migration:d<TAB> to mage migration:down and mage fmt migration:u<TAB> to mage fmt migration:up.

Not run: bash 4/5 interactively (the unit test simulates its word splitting), Windows, Linux.