fix: narrow several from any type assertions in opencode core

[kitlangton]

Summary

First pass at the 107 "Unsafe assertion from any" sites flagged by oxlint's typescript/no-unsafe-type-assertion rule. These are the most dangerous casts because they silently launder untyped values into typed ones.

Strategy: focus on real laundering of any values, not inline-typed .json() as {...} patterns (lint still flags those, but they're fine — they annotate the expected API shape).

Not enabling the rule yet — this is a down payment on a larger cleanup.

Changes

• util/filesystem.ts — change readJson<T = any> default to readJson<T = unknown>. Callers must now opt into a concrete type or handle unknown, which means a value parsed from a JSON file can't silently satisfy any shape.
• npm/index.ts — add a PackageJson type for readJson so pkg.dependencies etc. are typed instead of any.
• cli/cmd/tui/context/kv.tsx, test/config/config.test.ts — pass explicit type params to readJson at the call sites.
• cli/error.ts — replace data?.suggestions as string[] | undefined and data?.issues as Array<...> with Array.isArray(...) ? ... : [] narrowing. Defensive: now behaves correctly (returns empty array) if the underlying error payload has the wrong shape.
• lsp/lsp.ts — type client.connection.sendRequest<Symbol[]>("workspace/symbol", ...) and drop the intermediate as any annotations on the .then callbacks; return results.flat() directly instead of as Symbol[].
• provider/provider.ts — narrow options?.workflowRef / options?.workflowDefinition via typeof === "string" instead of casting the values out of Record<string, any>.
• session/session.ts — replace (... ?? 0) as number on the Anthropic/Bedrock/Vertex cache token fallback chain with Number(...) coercion. The existing safe() wrapper already handles NaN.
• tool/tool.ts — change Context.extra from { [key: string]: any } to { [key: string]: unknown }. Downstream casts like ctx.extra?.promptOps as TaskPromptOps now become "more narrow" warnings instead of "from any" errors.

Impact

• Unsafe assertion from any in opencode src: 29 → 25 (4 fixed).
• Total no-unsafe-type-assertion across the repo: 1079 → 1073 errors.

Most of the remaining "from any" sites in opencode src are .json() as {...} inline-type annotations for fetch responses, which is a reasonable pattern the lint will always flag. Future passes could add runtime schema validation for the most critical ones (well-known config, model registries, etc.).

Verification

• bun turbo typecheck passes (all 13 tasks).
• bun run lint passes (oxlint warnings only, no errors).
• bun test test/config/config.test.ts passes (73 tests, touches the updated readJson call).