feat(rust-server): Gemini 3.5 title model + head/tail trim for long first messages

[danshapiro]

What

Two changes to the Rust server's AI session titling in crates/freshell-server/src/ai_title.rs:

1. Model bump gemini-2.5-flash-lite → gemini-3.5-flash-lite. Evidence: identical prompt + identical production first-messages, 2.5 drew Agent Design / Agent Status style failures (and reproduced them on replay), 3.5 produced topic-first titles 5/5 and honest low-signal titles 3/3.
2. Prompt input windowing: a first user message over 2000 chars now contributes first 1000 chars + \n...[trimmed]...\n + last 1000 chars (char-counted, multibyte-safe) instead of a blind first-2000 prefix.

Node server/ is obsolete and deliberately untouched; e2e fake-Gemini pin (auto-title-rust.spec.ts) follows the new model path.

Evidence

• TDD red→green per task (route-404 red → const green; missing-marker red → helper green)
• npm run check (typecheck + coordinated full suite) green at HEAD 875e036
• e2e: auto-title-rust.spec.ts 5/5 on Cloud Run at commit-tagged image; also 5/5 local
• Independent reviews: plan PASSED (1 round, 2 doc nits fixed), whole-branch + final delta BOTH PASSED 1 round, zero findings
• Load-bearing pre-flight: falsified + corrected two plan assumptions (vacuous spec typecheck; cold e2e pre-warm requirement)

Refs: worktree .worktrees/title-gemini-35-trim, run records under .worktrees/.the-usual-logs/title-gemini-35-trim/