chore: close deferred work — real GeminiRunner, Local UI option, db.go cleanup

Closes the three items left on the deferred queue after the post-epic
cleanup.

GeminiRunner.execOnce now actually executes the gemini binary instead
of writing hardcoded stream data. Mirrors ClaudeRunner.execOnce:
- exec.CommandContext with the same env vars (CLAUDOMATOR_API_URL etc.)
- process group SIGKILL on context cancel
- stdout piped through parseGeminiStream → stdoutFile
- stderr to file
- exit codes captured, stderr tail surfaced on failure

Test infrastructure bug uncovered in passing: testServerWithGeminiMockRunner's
mock script used double-quoted echo with literal triple-backticks, which
bash interpreted as command substitution. The script always produced
empty output. The bug was invisible until now because GeminiRunner
ignored the script entirely. Switched to a single-quoted heredoc.

Frontend: index.html dropdown gains a "Local" option. No JS branching
needed — the value flows through to agent.type verbatim and downstream
display reads the type string as-is.

storage/db.go: removed stale debug-comment scaffolding (the "TODO:
Replace with proper logger" block) that was tracking a dead
`fmt.Printf` call. The path it commented on is fine without logging —
unmarshal errors are returned wrapped.

Test status: `go test -race ./...` green across every package, zero
skips, zero excluded tests.

https://claude.ai/code/session_017Edeq947TpSm1vQTxMhi1J

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