refactor(playground): derive buildHonoProxy mount prefix from label

[LissaGreense]

Summary

buildHonoProxy(prefix, upstream, label) took both prefix and label where, by construction at both call sites, prefix === \"/api/\" + label:

.all(\"/api/daemon/*\", buildHonoProxy(\"/api/daemon\", DAEMON_URL, \"daemon\"))
.all(\"/api/tunnel/*\", buildHonoProxy(\"/api/tunnel\", TUNNEL_URL, \"tunnel\"))

The invariant was implicit and unenforced. A mismatched mount (e.g. buildHonoProxy(\"/foo\", url, \"daemon\")) would silently strip /foo from the request path while still stamping x-forwarded-prefix: /api/daemon on the upstream — surfacing downstream as wrong external URLs (OAuth callbacks, etc.) with nothing failing loudly at the mount point.

Drop the prefix parameter; derive it inside the function as /api/\${label}. Single source of truth, invariant enforced by construction.

Also swap replace(new RegExp(prefix), \"\") for a literal startsWith + slice so the prefix can't be reinterpreted as a regex pattern (no exploit today with hardcoded labels, but the regex shape is a general footgun).

Closes #316.

Scope

• tools/agent-playground/src/lib/server/proxy.ts — signature change + regex → literal swap
• tools/agent-playground/src/lib/server/static-app.ts — both call sites updated
• tools/agent-playground/src/lib/server/proxy.hono.test.ts — existing 2 tests updated for new signature; added a third tunnel-label case that pins the x-forwarded-prefix / stripped-path single-source-of-truth property

Risks

• Behavioral: none under the only two call sites — both used the /api/\${label} invariant already, so the derived prefix produces the identical strip + forward header.
• Signature break: internal API only, no external consumers. The function is exported from one file and called from one other (and tests). `grep -rn buildHonoProxy` confirms.
• Regex → startsWith: identical behavior on the production inputs ("/api/daemon", "/api/tunnel"). Strictly safer because metacharacters in the prefix can no longer alter matching.
• Coordination: PR Move agent-playground → apps/studio-ui #293 is renaming `tools/agent-playground` → `apps/studio-ui`. Either order works — whichever lands first, the other rebases trivially (three small file edits, no semantic conflict).

Test plan

• `npx vitest run src/lib/server/proxy.hono.test.ts` — 3/3 pass (existing 2 + new tunnel-label case)
• `npx vitest run src/lib/server/` — 79 pass / 1 pre-existing unrelated flake in `mcp.test.ts` (Deno.openKv unavailable in vitest env, untouched by this diff)
• `deno check` clean on proxy.ts + static-app.ts