fix: plan-mode reminder ignored-flag is stale on cross-turn model switch (experimental flag)

Found during v0.8.3 release review (Chaos Gremlin / prompt-injection red-team persona). **Deferred** because it needs a design change to the J2 persisted-flag ownership contract and a cross-vendor test matrix (>30 min).

## Context
Under `OPENCODE_EXPERIMENTAL_PLAN_MODE=true`, `insertReminders` (in `packages/opencode/src/session/prompt.ts`) stamps `ignored: true` onto persisted reminder parts at **insertion time**, computed from the *inserting turn's* model via `isAnthropicLikeModel`. The flag is then persisted via `Session.updatePart`.

## Bug
If the user switches models with `/model` between turns, the persisted `ignored` flag reflects the *old* turn's model, not the current one:
- **GPT-5.x (turn 1) → Claude (turn 2):** the turn-1 reminder persisted with `ignored: true`. On turn 2 it's suppressed from Claude's user role by `toModelMessages` (gates on `!part.ignored`) **and** it's not in turn-2's freshly-built `trustedReminderParts`, so it isn't hoisted either → Claude silently loses that plan-mode constraint for the turn (degradation).
- **Claude (turn 1) → GPT-5.x (turn 2):** the turn-1 reminder persisted with `ignored` unset. On the GPT turn it slips back into the user role as a `` block → re-triggers the original #887 refusal class.

Both directions are flag-gated (default path unaffected, since default-path reminders are in-memory only and rebuilt each turn) and the GPT→Claude case is degradation-only, so this is **not** a default-path regression. But the experimental path ships a latent turn-ordering bug.

## Suggested fix
Compute the hoist/`ignored` decision at **read time** per turn from the *current* turn's model, rather than baking it into the persisted row at insert time; or re-evaluate persisted reminder parts each turn against the active model. Add tests across the switch matrix (GPT→Claude, Claude→GPT, Gemini→GPT) under the experimental flag.

## Scope
`packages/opencode/src/session/prompt.ts` (`insertReminders` experimental branch), `packages/opencode/src/session/message-v2.ts` (`toModelMessages` ignored-gate).