fix: add ReviewFixer to builtin session primary agent whitelist (#2124)

[xielixing]

Problem

Issue #2124: Starting the Review Fix phase fails with Unknown session mode: ReviewFixer.

PR #2106 fixed the first half of the external primary agent profile resolution issue — CodeReview and DeepReview were blocked at session creation because is_builtin_session_primary_agent did not whitelist them. However, the fix phase was missed: ReviewFixer (submitted by DeepReviewActionBar.handleStartFixing → sendMessage(..., 'ReviewFixer', 'agentic')) is still not in the whitelist.

Root Cause

src/crates/assembly/core/src/agentic/agents/registry/external.rs — is_builtin_session_primary_agent only matched CODE_REVIEW_AGENT_TYPE and DEEP_REVIEW_AGENT_TYPE, not REVIEW_FIXER_AGENT_TYPE.

When the frontend submits agentType='ReviewFixer', the coordinator calls resolve_primary_agent_for_workspace which hits is_builtin_session_primary_agent. Since ReviewFixer is not whitelisted, it falls through to the external-subagent resolution path and fails with Unknown session mode: ReviewFixer.

Fix

Add REVIEW_FIXER_AGENT_TYPE to the is_builtin_session_primary_agent whitelist, so ReviewFixer resolves through the local primary-agent path for create, turn, restore, and compaction — same as CodeReview and DeepReview.

Changes

1. external.rs — Import REVIEW_FIXER_AGENT_TYPE and add it to the is_builtin_session_primary_agent match. Update doc comments on is_builtin_session_primary_agent and is_local_session_primary_entry.

2. tests.rs — Add "ReviewFixer" to builtin_review_agents_resolve_as_local_session_primaries and local_route_resolves_review_agents_as_session_primaries.

3. coordinator.rs — Add "ReviewFixer" to review_agent_child_sessions_create_successfully.

Validation

• cargo check -p bitfun-core --no-default-features --features agent-runtime — pass
• cargo test ... builtin_review_agents_resolve_as_local_session_primaries — 1 passed
• cargo test ... local_route_resolves_review_agents_as_session_primaries — 1 passed
• cargo test ... review_agent_child_sessions_create_successfully — 1 passed (with canvas-runtime)

Related

• Fixes Review 修复阶段失败：Failed to start dialog turn: Unknown session mode: ReviewFixer（#2106 修复不完整） #2124
• Supersedes incomplete fix in fix(review): restore CodeReview/DeepReview session creation and turns #2106

[xielixing]

Closing as duplicate of #2126, which implements the identical fix (adding REVIEW_FIXER_AGENT_TYPE to is_builtin_session_primary_agent whitelist). Please merge #2126 instead.