fix(compaction): preserve OpenCode session attribution

[Altairpaca]

Summary

Fixes #1523.

Builtin compaction generates its own provider request through ModelRuntime.stream() rather than the normal AgentSession stream wrapper. That bypasses the OpenCode session-attribution layer in sdk.ts, so an opencode / opencode-go summarization request can reach the provider without x-opencode-session and fail with 400 MissingSessionID even while ordinary turns succeed.

The patch reuses the existing OpenCode provider/host predicate at the compaction request boundary. It deliberately does not move session attribution into ModelRuntime, change provider routing/auth, or special-case DeepSeek: the failure is a request-boundary omission that affects any model sent through OpenCode Go.

Header contract

The compaction request preserves the same authority order as normal AgentSession traffic:

1. generated OpenCode routing defaults (x-opencode-session, x-opencode-client);
2. resolved auth/configured request headers;
3. the existing provider-request / before_provider_headers transform.

Explicit configuration and extensions therefore remain authoritative.

Changes

• Rename/export the narrow helper as getOpenCodeSessionHeaders() so its scope is explicit and the existing opencode / opencode-go / opencode.ai predicate is reused rather than duplicated.
• Seed builtin summary request headers from the active SessionManager id before the existing auth/header transform.
• Document at the call site why compaction must do this locally: it bypasses sdk.ts's AgentSession stream wrapper.
• Keep the regression in test/suite/regressions/ and pin provider-id routing, opencode.ai host routing, auth/configured-header override precedence, extension override precedence, and non-OpenCode neutrality.
• Add canonical exact-nearest tracker entries in src/core/changes.md and src/core/extensions/builtin/compaction/changes.md for the two upstream-owned production paths.

Scope / upstream sync

The branch is synchronized with upstream main at 464f1a54e8569cdfe8e4620583e19d594a6b96e9 (PR head 27fcf328cde6c1ec015cdbe76f07913f3c421840). Current main still has the original private getSessionHeaders() and compaction still forwards only auth/configured headers, so the defect remains present upstream.

The semantic runtime diff remains two production files plus one focused regression; the two additional changed files are repository-policy changes.md trackers only.

No retry policy, payload transformation, credential selection, ordinary AgentSession request behavior, or non-OpenCode provider behavior is intentionally changed.

Verification status

The regression is deterministic and does not call a real provider. The exact-nearest tracker requirement is now satisfied in the branch.

This connector environment cannot execute the repository test runner, so I am not claiming a local green run. PR-triggered CI / Changelog workflows for fork contributions may require repository approval before jobs execute; action_required is an approval gate, not a test result. Any executable type/test or changelog-policy failure remains blocking evidence.

Per CONTRIBUTING.md, release CHANGELOG.md entries are maintainer-owned; this PR does not edit them. The PR stays draft until executable CI evidence and the release-note policy are accounted for.

[Altairpaca]

Audit/update pass complete on the runtime patch. I tightened the helper name to the OpenCode-specific getOpenCodeSessionHeaders(), documented the alternate compaction request boundary, moved the regression into test/suite/regressions/, and expanded it to pin provider-id + host detection and both override layers (resolved/configured headers, then before_provider_headers). The branch is also resynced to current main at e66c1553a60917260f480ebab09c817fda73fb14.

I am intentionally keeping this as a draft for the moment. Two evidence gates remain: exact-nearest changes.md entries for the two upstream-owned production paths, and executable CI (the PR workflows have been action_required, so there is no test result to claim yet). Per CONTRIBUTING.md I will not add the release CHANGELOG.md entry from the external-contributor side. No provider/network test was used, and I am not claiming local green from this connector environment.

[Altairpaca]

Follow-up on the draft: the exact-nearest changes.md coverage for both upstream-owned production paths is now committed on head 27fcf328cde6c1ec015cdbe76f07913f3c421840. The runtime/test scope is unchanged; the PR now has five changed files because the two additional files are policy trackers only. CI and Changelog gate are again stopped at action_required before jobs are created, so executable verification still needs repository approval.