refactor(#1148): join Claude command parts

[GHX5T-SOL]

Summary

• Build the Claude CLI invocation from explicit command parts joined with single spaces.
• Remove the convention where optional flag fragments have to carry trailing spaces.
• Add regression tests for all optional flags and no optional flags to catch missing or doubled separators around --agent.

Closes #1148

Validation

• go test ./internal/cli -run 'TestBuildClaudeCommand'\n- go test ./internal/cli\n- go vet ./internal/cli\n- go test ./...\n- git diff --check\n- git diff | gitleaks stdin --no-banner --redact --timeout 30\n\nLint limitation: make lint failed locally because pre-commit is not installed on this host. I also tried uv run --with pre-commit python -m pre_commit run; it initialized hooks but failed before completing because the lint-md-links hook requires the lychee executable, which is not installed locally.\n

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Commit: 5fe0cafffa9f6d4c754b9babb6182227bb94d0bc