feat(models): add Claude Opus 5 and repoint the opus alias to it

Adds `claude-opus-5` to models.json (the SPOT) and moves the `opus` alias
from `claude-opus-4-8` to it. `/v1/models` goes 6 -> 7; OpenClaw picks the
new entry up on the next `ocp update` via scripts/sync-openclaw.mjs.

server.mjs is NOT modified by this PR — models.json is the single source of
truth (ADR 0003) and every consumer (server.mjs MODEL_MAP, setup.mjs,
sync-openclaw.mjs, ocp-connect) derives from it. ocp-connect needs no change
either: its metadata table matches on the `claude-opus` FAMILY prefix, which
`claude-opus-5` hits (the guard added in PR #152 review).

Alignment evidence. The claim "the CLI itself defaults opus -> claude-opus-5"
is verified from the compiled CLI binary 2.1.220 via `strings`, the same
protocol used for #152/#168:

    latest_per_family:{fable:"claude-fable-5",opus:"claude-opus-5",
                       sonnet:"claude-sonnet-5",haiku:"claude-haiku-4-5"}

The same registry entry gives context:{window:1e6,native_1m:true},
max_output_tokens:{default:64000,upper:128000} and pricing:"tier_5_25" --
i.e. identical $5/$25 per MTok to Opus 4.8, so the alias repoint carries no
cost change. Availability confirmed with a live subscription-pool completion
(`claude -p --model claude-opus-5` -> "OK"). This mirrors #168 (sonnet ->
sonnet-5), which established the precedent that following the CLI's own
latest_per_family is the correct default behavior.

contextWindow is deliberately 200000, not the native 1M:

1. MAX_PROMPT_CHARS is a SINGLE GLOBAL budget, not per-model.
   derivePromptCharBudget (lib/prompt.mjs) returns
   `max(floor, max(...contextWindows) * 3)` across ALL entries, so a 1M entry
   would lift the truncation ceiling to 3,000,000 chars for
   claude-haiku-4-5-20251001 as well -- genuinely a 200k-native model --
   turning clean OCP-side truncation into an upstream API rejection.
2. OpenClaw scales its history budget linearly off this value:
   contextWindow * maxHistoryShare * SAFETY_MARGIN (= x0.6), plus an
   oversized-message threshold at x0.5 (compaction-planning, OpenClaw
   2026.7.1). Its own bundled registry hardcodes 200000 for Claude models,
   and the upstream request to raise it to 1M (openclaw#22979) was closed
   "not planned" -- 1M needs a beta header its compaction path omits.

Raising the window for real requires per-model budgets and is ADR-level;
a new regression test pins the invariant so that change has to be deliberate.

Tests: 452 passed, 0 failed. Three mutations verified to bite:
  - revert aliases.opus -> 4-8            => 1 failure (opus-alias SPOT)
  - delete the claude-opus-5 entry        => 2 failures (presence + dangling alias)
  - set its contextWindow to 1000000      => 2 failures (invariant + budget SPOT)

E2E on an isolated port (:3999, prod on :3456 untouched and verified so):
/v1/models returns 7 ids with claude-opus-5 first; a request with
model:"opus" logs event=claude_spawned model=claude-opus-5 tier=opus and
returns a real completion. Derived MAX_PROMPT_CHARS stays 600000.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017gbqUZ8HfBZpjjbzQ85oH8

README.md

@@ -113,11 +113,11 @@ node setup.mjs
 
 `setup.mjs` verifies the Claude CLI, starts the proxy on port 3456, and installs auto-start (launchd on macOS, systemd on Linux). The `ocp` CLI lands at `~/ocp/ocp` — symlink it onto your PATH (`sudo ln -sf ~/ocp/ocp /usr/local/bin/ocp`, or `ln -sf ~/ocp/ocp ~/.local/bin/ocp`) or alias it (`alias ocp=~/ocp/ocp`); the rest of the docs assume `ocp` is on your PATH.
 
-**Verify** — should list 6 models:
+**Verify** — should list 7 models:
 
 ```bash
 curl http://127.0.0.1:3456/v1/models
-# claude-opus-4-8, claude-opus-4-7, claude-opus-4-6, claude-sonnet-5, claude-sonnet-4-6, claude-haiku-4-5-20251001
+# claude-opus-5, claude-opus-4-8, claude-opus-4-7, claude-opus-4-6, claude-sonnet-5, claude-sonnet-4-6, claude-haiku-4-5-20251001
 ```
 
 **Connect one IDE** — point any OpenAI-compatible tool at the proxy, then reload your shell and start a tool (Cline / Continue / Cursor / OpenCode):
@@ -169,8 +169,9 @@ Any tool use happens server-side, under the `--allowedTools` set configured on t
 
 | Model ID | Notes |
 |----------|-------|
-| `claude-opus-4-8` | Most capable (default for `opus` alias) |
-| `claude-opus-4-7` | Previous Opus, retained for pinning |
+| `claude-opus-5` | Most capable (default for `opus` alias) |
+| `claude-opus-4-8` | Previous Opus, retained for pinning |
+| `claude-opus-4-7` | Older Opus, retained for pinning |
 | `claude-opus-4-6` | Older Opus, retained for pinning |
 | `claude-sonnet-5` | Latest Sonnet (default for `sonnet` alias) |
 | `claude-sonnet-4-6` | Previous Sonnet, retained for pinning |