Decompose the mega files: lib.rs (18.7k), config.rs (12.3k), client.rs (11.1k), runtime_threads.rs (9.3k)

## Combined Core execution: C09

[Core execution plan]() owns order and scope. This issue contributes to C09; the linked plan owns the complete packet and any outstanding candidate acceptance. Record this issue's path claims and implementation evidence here.

**Issue mapping:** [SHA-6291 / GitHub #5586]()
covers cohesive large-file decomposition across C03–C09; move whole behaviors
and owning tests, with checked dependency boundaries rather than line-count
targets. [SHA-6286 / GitHub #5580]() moves remaining
event handling into domain reducers and leaves dispatch in `event_loop.rs`;
preserve the existing reducer pattern and migrate touched arms completely.
[SHA-6287 / GitHub #5581]() supplies the per-call
telemetry consumer requirements implemented in C11 with #6011: live token/time
display, cache visibility and durable mid-turn accounting, with authoritative
completion reconciliation and no double counting. Credit already-live cost,
background cost, timing and context surfaces.

[SHA-5509 / GitHub #3957]() separates shared modal
infrastructure from owned Config/SubAgents views and makes all provider
credential entry use the same masking, validation and geometry primitive.
Prove paste/delete/Enter/Esc/failure/retry and 80x24/120x32 wrapping; secrets
must never reach screenshots, logs or errors.
[SHA-5506 / GitHub #3954]() splits setup by
constitution, runtime, provider, persistence/report and navigation ownership
where warranted; preserve constitution checkpoints and onboarding handoff.
These client refactors keep behavior and private-test visibility intact.

[SHA-6251 / GitHub #5528]() requires visible
workflow dispatch/schema failures even before a child starts, with preserved
error/status receipts over the shared event authority.
[SHA-6156 / GitHub #5268]() requires visible queue,
send-now, cancel/keep-draft and named waits over existing runtime channels;
qualify those states with C12 and C08's submitted-steer consume/ack contract.
C01's #5479 register and controls must use this same runtime projection.

**Owner: client-adapter owner. Dependencies: C08.** Fold the embedded HTTP
API into the existing app-server HTTP/SSE/stdio surface over the shared
runtime, preserving routes and authentication. Move argument parsing and
headless presentation from TUI `lib.rs` into CLI; preserve binary names and
public commands. TUI owns terminal lifecycle, rendering, input, pickers and
slash-command presentation, calling the same runtime operations. Use existing
in-process `EngineHandle`/`Op`/`Event` seams; local terminal use need not hop
through HTTP. Separate wire DTOs from internal reply channels/capabilities.

**Completion evidence required:** all three clients reach the same lifecycle/execution owner,
transport/auth/protocol parity and reconnect/recovery pass, no UI-owned
provider I/O/policy/store/loop remains, and obsolete dependencies and temporary
internal shims are removed. Adopt `core/protocol_parity.rs` in real consumers
or retire it; unused parity code is not convergence. Resize UI files only by
remaining responsibility. Apps product work is not part of this packet.

---

## Original issue and contributor history

User ask for the 0.9.12 cleanup lane: the 10k+-line files keep causing pain. Measured today (prod code, excluding the 20k-line test files which should split alongside their subjects):

* `crates/tui/src/lib.rs` — 18,747
* `crates/tui/src/config.rs` — 12,346
* `crates/tui/src/client.rs` — 11,122
* `crates/tui/src/runtime_threads.rs` — 9,322
* `crates/tui/src/core/engine.rs` — 6,891, `core/engine/turn_loop.rs` — 6,668, `runtime_api.rs` — 6,542, `tui/ui/event_loop.rs` — 5,721 (event_loop already tracked in #5580)

Approach (same discipline as #5580): mechanical module extraction, one cohesive region per commit, no behavior change, moves verified by `cargo check` + the file's existing test slice. Suggested first cuts: [client.rs]() wire-parsing (parse_usage/SSE chunk parsing + their tests) into `client/wire.rs`; [config.rs]() merge/persistence into `config/merge.rs`; [lib.rs]() engine-config assembly into its own module. Declared one-way per AGENTS.md once started: new code lands in the extracted modules, touched regions migrate.

Tracker: #5573. Related: #5580, #5581.

Read AGENTS.md and the C09 core execution plan first, then inspect crates/tui/src/client.rs, config.rs, lib.rs, and runtime_threads.rs. For a bounded starting slice, trace parse_usage and SSE chunk parsing with their existing tests, then run cargo check and that file's test slice. Done requires behavior-preserving module extraction with dependency boundaries and the stated lifecycle, protocol, reconnect, and recovery evidence.

Written by the indexing model from the issue text.