refactor(sessiontitle): simplify Generator without changing behavior

[dgageot]

Simplifies pkg/sessiontitle to make it easier to read and maintain. No public API changes, no behavior changes — same fallback chain, same timeout, same prompts, same observable outputs to callers.

Changes

Two refactor-only commits, kept separate so the diff stays easy to follow.

1. refactor(sessiontitle): tidy Generator without changing behavior

• Consolidated the three early-return guards (g == nil, no models, no user messages) at the top of Generate, before allocating the timeout context.
• Replaced the manual two-pass nil-filter in New with a single slices.DeleteFunc.
• Extracted generateOnce so each attempt closes its stream via defer (no more streamErr plumbing).
• Extracted buildPrompt to keep prompt construction out of the main loop.
• Removed the dead if baseModel == nil { continue } (New already filters nils).
• Tightened sanitizeTitle to a single-line loop body.

2. refactor(sessiontitle): unify the per-attempt failure path

• Pushed the "empty model output" check into generateOnce: it now returns a normal error like any other attempt failure, so the main loop has a single failure path.
• Extracted drainStream as a small helper that owns reading and closing the message stream.
• Unified per-attempt log level at Debug (these aren't fatal while we have fallbacks; callers already log/wrap the final error).
• Dropped the post-loop if lastErr != nil guard — the loop's invariants now make lastErr always non-nil at that point.

Final shape

The package is now a clear linear pipeline of small, single-purpose functions:

• New — build the model list (filtering nils)
• Generate — iterate models, log per-attempt outcomes, return the first success or wrap the last failure
• generateOnce — clone provider, open stream, drain, sanitize, validate non-empty
• drainStream — read + close
• buildPrompt — format messages
• sanitizeTitle — first non-empty line, strip \r

Validation

• mise lint — clean (golangci-lint: 0 issues; custom analyzer: no offenses; go mod tidy clean)
• mise test — full suite passes, including pkg/sessiontitle, pkg/app, pkg/server, pkg/runtime, cmd/root
• Existing tests (TestGenerator_Generate_FallsBackOnStreamCreateError, TestGenerator_Generate_FallsBackOnRecvError, TestGenerator_Generate_FallsBackOnEmptyOutput) all still pass unchanged.