fix(custom-model): root-cause and fix DeepSeek's HTTP_404 (missing /v1)

DeepSeek Harness's own bundled provider module
(@deepseek-ai/dsh-llm-deepseek) builds its request URL as
`${DEEPSEEK_BASE_URL}/chat/completions` with no `/v1` insertion of its
own (its real public API, https://api.deepseek.com, expects the
caller's base URL to already carry any needed prefix), while
llama-swap/llama.cpp only ever serves the OpenAI-conventional
`/v1/chat/completions`.

Confirmed two ways:
- Installed the real @deepseek-ai/dsh package (all its actual
  published dependencies) into a scratch dir purely to read
  dsh-llm-deepseek's source: `fetch(`${connection.baseURL}/chat/
  completions`, ...)`, baseURL read straight from DEEPSEEK_BASE_URL —
  the same grep-the-real-source bar pi/grok's fixes were held to.
- Live against the test-picker's llama-swap: `POST <baseUrl>/chat/
  completions` -> 404, `POST <baseUrl>/v1/chat/completions` -> 200,
  same endpoint. dsh's own error template ("DeepSeek API error (HTTP
  ${status})") reproduces the originally-reported
  "dsh: HTTP_404: DeepSeek API error (HTTP 404)" exactly.

- New registry field `appendV1Suffix` (env kind only, deepseek's entry
  alone — claude/gemini must NOT get it, since claude was already
  confirmed working against the unmodified baseUrl). When set,
  buildCustomModelInjection runs endpoint.baseUrl through the same
  withV1Suffix() helper configDir-kind CLIs (pi/grok/codex) already
  use, instead of writing it verbatim.

Not yet re-run end-to-end through a real dsh binary — no install
available in this environment (not in PATH, and the test-picker
container doesn't bundle it) — so this is source-confirmed and
live-verified at the HTTP level, not yet promoted to "verified"
alongside claude/opencode/pi/grok/omp. Docs (custom-model-endpoints.md,
the plan doc's confidence table, the wiki page, CLAUDE.md) all updated
to reflect this precisely rather than leaving the old "root cause not
identified" claim in place.

2 new/updated tests for the /v1 suffix (including idempotency against
a baseUrl that already ends in /v1) plus a corrected mock-server
contract test. Typecheck/lint clean; full suite shows no new
regressions.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RqZeHrRS6DYcGcGX2p9EwG

src/config/cli-registry/schema.ts

@@ -364,6 +364,11 @@ const capabilitiesSchema = z
           // same file to pre-seed the state a real, already-onboarded profile carries. See
           // the customModelInjection doc comment in cli-registry/types.ts.
           skipFirstRunPrompts: z.boolean().optional(),
+          // DeepSeek-only, confirmed by reading its own bundled SDK source: it concatenates
+          // "/chat/completions" onto baseUrlVar's value with no "/v1" of its own, while
+          // llama-swap/llama.cpp only serves the "/v1/..." path — claude/gemini must NOT
+          // get this. See the customModelInjection doc comment in cli-registry/types.ts.
+          appendV1Suffix: z.boolean().optional(),
         })
         .strict(),
       z