providers/gemini + loop: next-speaker check and stall recovery for Gemini

[erain]

Goal

P2.7 of docs/coding-harness-roadmap.md: Gemini's classic stall — narrating "Next, I will…" and stopping without a tool call — plus residual reliability gaps not covered by v1.8.0's thoughtSignature work.

Scope

• Next-speaker heuristic when a turn ends with no pending tool calls: fast paths first (last message functionResponse → continue; empty model turn → continue), optional cheap-model JSON check; on "model", auto-send "Please continue." bounded by max-turns.
• Sanitize lone UTF-16 surrogates in outbound text (Gemini API crash).
• Over-escape repair for tool args lands with tools/fs: edit_file repair ladder — flexible matching, instructive errors, escape repair #338; this issue wires any remaining provider-side cases.

Out of scope

Flash model fallback chains; quota-tier policies.

Implementation notes

Sources: Gemini CLI nextSpeakerChecker.ts (3-rule prompt + fast paths), geminiChat.ts. The auto-continue belongs in the loop with a provider-capability flag so it can also serve open-weight models that exhibit the same stall.

Docs update required

CHANGELOG; provider-guide note.

Verification commands

go test ./providers/gemini/ ./loop/ && go build ./... ; GEMINI_API_KEY=... go test ./providers/gemini -run Live

Acceptance criteria

Fake-provider test: a no-tool "I will now…" turn triggers exactly one "Please continue." and the loop completes; surrogate sanitization unit-tested; live smoke unaffected.

[erain]

Shipped in PR #351. Opt-in next-speaker stall recovery: deterministic closing-sentence intent check, bounded 'Please continue.' nudges (2/run, marked glue/auto-continue, EventAutoContinue), enabled automatically for gemini in cmd/glue. Surrogate sanitization verified unnecessary per #313.