feat(convo): emit model on conversation.append; kind agent-coding-session v1.2.0

[akesling]

Reported by Pathbase: empathic/pathbase#225 — consumers reading structural.model on conversation.append always get null, because the model is encoded only in the actor string (agent:<model>) and meta.actors.

Change

• Emit model on conversation.append (toolpath-convo::derive): the structural payload carries the model that produced the turn whenever the source session records one; the key is omitted otherwise — no "unknown" sentinel in payloads. Single emitter, so all seven providers (claude, gemini, codex, pi, cursor, opencode, copilot) gain it at once. Actor strings (including agent:unknown) and meta.actors are unchanged.
• Extraction round-trips it: extract now prefers the payload model and falls back to parsing the agent:<model> actor for pre-v1.2.0 documents.
• Kind agent-coding-session v1.2.0: new bundled schema documenting the optional model property on conversationAppend (only semantic delta from v1.1.0), registered in BUNDLED_KINDS; PATH_KIND_AGENT_CODING_SESSION now stamps the v1.2.0 URI with the previous URI preserved as ..._V1_1_0; site spec page + indices updated. Consumers that match the kind by slug/major (pathbase does) are unaffected.
• Crate versions bumped in-commit per the kind-URI-change policy established by the v1.1.0 adoption: toolpath 0.8.0, toolpath-convo 0.12.0, toolpath-claude 0.13.0, remaining derivers and CLIs accordingly; Cargo.lock, site/_data/crates.json, and the toolpath-cli shim kept in sync. All bumps are breaking-level under 0.x caret semantics, so downstream pins do not auto-flow.

The companion read-side change (model resolution with actor-def and actor-suffix fallbacks, which also fixes already-uploaded documents retroactively) is empathic/pathbase#304.

Testing

Full quality gate green: cargo test --workspace (1786 tests) with -D warnings, clippy (workspace + all-targets), fmt, rustdoc -D warnings, path p validate examples/*.json, site build (12 pages, v1.2.0 schema materialized), shim cargo check. New tests pin: payload model present when the session records one, absent otherwise (actor stays agent:unknown), payload preferred over actor parse on extract, v1.2.0 bundled/newest-resolution alongside exact-version pinning of v1.0.0/v1.1.0.

---

^{Need help on this PR? Tag /codesmith with what you need. Autofix is disabled.}

[github-actions]

🔍 Preview deployed: https://40a73a38.toolpath.pages.dev