docs: fix flag punctuation and example formatting (quick wins)

Description

A consolidated "quick wins" MR for the small punctuation and formatting findings from the style guide audit in #8263 (closed).

• cluster agent get-token and cluster agent update-kubeconfig: adds a trailing period to the --cache-mode flag description (fixed once in the shared agentutils helper).
• runner update: adds trailing periods to the --pause and --unpause flag descriptions.
• issue list and incident list: capitalizes the --in flag description (fixed once in the shared issuable/list builder).
• stack infer: fixes the Examples section so it renders as a real code block. The per-example comments used ##, which broke the generated Markdown; they now use single-# shell comments, and the command lines drop the $ prompt prefix to match the rest of the docs.

No behavior changes — descriptions and example formatting only.

Pages updated:

• cluster agent get-token, cluster agent update-kubeconfig
• runner update
• issue list, incident list
• stack infer

Content follows the documentation style guide and the GitLab CLI (glab) documentation style guide.

Related issues and merge requests

• docs: Fix CLI documentation inconsistencies ide... (#8263 - closed)

Documentation

If this MR adds or updates CLI command documentation, follow the conventions in the GitLab CLI (glab) documentation style guide, run make gen-docs, and commit the updated files in docs/source/.