fix(cosh): filter unavailable agents in key sharing prompt

[samchu-zsl]

Description

Fix the Custom Provider auth flow so the Agent Key Sharing prompt only
offers agents whose config directory actually exists on the host.

Previously the share list was hard-coded to OpenClaw and Qwen Code,
and the flow was gated by a single "any agent config dir present"
boolean. When only one of the two was installed, users still saw the
missing agent in the list (the exact scenario reported in #386 —
~/.qwen is absent but the Qwen Code option is shown anyway). The
auto-skip to manual config also required both agents to fail, so a
user with only one installed agent whose key could not be read got
stuck re-selecting the same agent instead of falling through to manual
entry.

Per-agent detection results (hasOpenclawDir, hasQwenCodeDir) are
now computed once on entering the USE_OPENAI flow and used both to
filter the CustomAgentKeySharePrompt (missing agents are added to
excludedChoices) and to gate the idle → done transition (all
detectable agents failed, not all agents). File I/O behavior is
unchanged: directory probing still uses existsSync only, no config
files are read until the user picks an agent.

Related Issue

closes #386

Type of Change

• Bug fix (non-breaking change that fixes an issue)

Scope

• cosh (copilot-shell)

Checklist

• I have read the Contributing Guide
• My code follows the project's code style
• I have added tests that prove my fix is effective or that my feature works
• For cosh: Lint passes, type check passes, and tests pass
• Lock files are up to date (package-lock.json / Cargo.lock)

Testing

cd src/copilot-shell
npm run format && npm run build && npm run lint && npm run typecheck && npm run test
# format OK, build OK, lint OK, typecheck OK
# cli:  Test Files 218 passed, Tests 3421 passed | 7 skipped
# core: Test Files 178 passed, Tests 3858 passed | 2 skipped
# Added CustomAgentKeySharePrompt.test.tsx (4 tests) covering the
# excludedChoices filter for missing agents.

Additional Notes

[gemini-code-assist]

Important

Installation incomplete: to start using Gemini Code Assist, please ask the organization owner(s) to visit the Gemini Code Assist Admin Console and sign the Terms of Services.

[samchu-zsl]

Caution

This PR was generated and submitted by AI.
Please review all changes carefully before merging. Pay special attention to logic correctness, edge cases, and potential side effects.

[samchu-zsl]

Manual testing successful:

1. If ~/{.openclaw, .qwen} does not exist, key sharing is skipped automatically
2. If only one of .openclaw/.qwen exists, only existing agent entries are shown
3. If both exist, both are displayed together

Confirmed the fix has no impact on other code logic.

[gemini-code-assist]

Important

Installation incomplete: to start using Gemini Code Assist, please ask the organization owner(s) to visit the Gemini Code Assist Admin Console and sign the Terms of Services.

[kongche-jbw]

lgtm

[kongche-jbw]

Consider deduplicating excludedChoices — using a Set prevents any future edge case.