[Feature] Add Gemini CLI as an agent install target

[Tasfia-17]

What problem are you trying to solve?

CONTRIBUTING.md lists gemini as an accepted, in-progress agent target
but no PR exists yet. Developers using Gemini CLI have no way to install
the TestSprite verification skill into their agent setup. Every other
supported agent (claude, cursor, cline, kiro, windsurf, copilot, codex)
has a working install target; gemini is the only explicitly accepted one
that is missing.

Proposed solution

Add gemini as a managed-section target that writes a sentinel-delimited
section into GEMINI.md in the project root. Gemini CLI reads GEMINI.md
as always-on project context, similar to how codex reads AGENTS.md.

Using managed-section mode (same as codex) ensures existing user content
in GEMINI.md is never clobbered. Both skills aggregate into one section.

New CLI surface:
testsprite agent install --target=gemini
testsprite setup --agent=gemini

Alternatives considered

own-file mode (CLI owns the whole file): rejected because both
testsprite-verify and testsprite-onboard resolve to the same GEMINI.md
path, causing a conflict error on the second skill write. managed-section
avoids this cleanly.

Scope check

• This request is about the CLI itself (commands, flags, output, ergonomics), not server-side product behavior.
• I have searched existing issues and this is not a duplicate.

[Tasfia-17]

/assign

[testsprite-hob]

Assigned to @Tasfia-17. Thanks for taking this on. (1 more slot available.)

Your open assigned issues (2/3):

• [Hackathon] setup should not re-prompt for API key when credentials already exist #206 — [Hackathon] setup should not re-prompt for API key when credentials already exist