feat(session-title): add fallback and model providers

packages/session-title/session-title-llm/README.md

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+# @deepseek-ai/dsh-session-title-llm
+
+Shared implementation policy for model-backed session-title providers. It resolves the auxiliary route, frames exact selected human messages as JSON, applies a language-aware title instruction, enforces input and output budgets, composes timeout and caller cancellation, assembles the stream, and returns normalized text with exact source seqs and model provenance.
+
+This package is a library, not a Cordis plugin. The provider plugins call `registerSessionTitleLlmProvider()` with their cadence and message selector; it validates shared config and delegates each revision to `generateSessionTitleWithLlm()`, so registration, route, prompt, cancellation, and validation behavior cannot drift between them.
+
+## Route and failure contract
+
+`provider` and `model` overrides are optional but must be supplied together as non-empty strings. Without that pair, the helper uses the exact provider/model route captured from the current session's logged `request/header`; an explicit refresh before any route exists therefore needs overrides. Input exceeding `maxInputBytes` rejects instead of being truncated. Timeout, cancellation, malformed or empty output, tool calls, and non-stop finish reasons also reject; the session-title service decides whether that rejection is an automatic warning or an explicit caller failure.
+
+## Configuration
+
+Every field is required except the paired route override; there are no library defaults.
+
+| Key | Contract |
+|---|---|
+| `targetWords` | Positive target word count for non-CJK titles. |
+| `targetCjkCharacters` | Positive target character count for Chinese, Japanese, or Korean titles. |
+| `maxInputBytes` | Positive aggregate UTF-8 byte ceiling across selected messages. |
+| `maxOutputTokens` | Positive auxiliary generation token cap. |
+| `timeoutMs` | Positive end-to-end deadline within the runtime timer limit. |
+| `provider`, `model` | Optional explicit route; both or neither. |
+
+## Model Experience
+
+### Auxiliary title request
+
+#### What the model sees
+
+The title model receives a fixed system instruction to return one concise unadorned title in the input language, including the configured word and CJK-character targets. Its one user message contains a JSON array of the exact selected human messages and their seqs.
+
+#### Token effect
+
+The auxiliary request consumes tokens according to selected input size and `maxOutputTokens`. It is separate from the main agent request and does not add title text or framing to agent history.
+
+#### KV Cache effect
+
+No main-request invalidation. Auxiliary cache reuse is provider-specific; the fixed instruction is reusable while the JSON message array changes with each revision.
+
+## Known Limitations and Deferred Work
+
+- The helper accepts text output only and rejects tool calls; structured-output adapters and provider-specific prompt variants are not exposed.
+- It enforces a byte ceiling for the whole selected input rather than clipping individual messages or applying a retention policy.