Feat/add codex support

[zvictor]

This PR is my contribution to #14 .
It's a continuation of it, so I am opening a PR both at the author's repo and here - in case he is not rresponsive there.

My changes:

• resolved conflicts
• added Codex support to update-agent-context in both powershell and shell
• made the fallback generic

[zvictor]

for those wanting to try this before PR approval:

uvx --from git+https://github.com/zvictor/spec-kit.git@feat/add-codex-support specify init --here

[zvictor]

testing it here I noticed that the scripts selection is broken.

Selected AI assistant: codex
Selected script type: sh
Initialize Specify Project
├── ● Check required tools (ok)
├── ● Select AI assistant (codex)
├── ● Select script type (sh)
├── ● Fetch latest release (release v0.0.30 (21,557 bytes))
├── ● Download template (spec-kit-template-copilot-ps-v0.0.30.zip)

spec-kit-template-copilot-ps-v0.0.30.zip should not be the downloaded template.

[DenWin]

@zvictor As far as I understand, a release providing the spec-kit must be generated first; otherwise, it falls back to spec-kit-template-copilot-*.

[localden]

Thanks for the contribution, @zvictor - #14 merged, so at this time there is Codex support.