Remove backward-compatibility fallbacks for unknown agent types

[gtrrz-victor]

Summary

• Remove DefaultAgentType constant and isSpecificAgentType() helper from strategy/common.go
• Simplify resolveAgentType() to use state or context agent type directly (no fallback to "Agent")
• Simplify ResolveAgentForRewind() to call agent.GetByAgentType directly (no default-agent fallback)
• Remove backfill logic in manual_commit_hooks.go that replaced "Agent" with the real type
• Remove TestInitializeSession_BackfillsUnknownAgentType test and update ResolveAgentForRewind tests

Agent type tracking was added 6 days after repo creation (Jan 9, 2026) and shipped in v0.3.5 (Jan 15). There are no realistic production sessions without agent type info, so these backward-compat patterns just add complexity.

AgentTypeUnknown itself is kept — it's used in switch cases for format-detection (treating unknown transcripts as JSONL), which is correct behavior for old checkpoints.

🤖 Generated with Claude Code

[cursor]

PR Summary

Medium Risk
Behavior changes in checkpoint rewind and session initialization may cause previously tolerated missing/unknown agent metadata to error instead of falling back, potentially breaking edge-case/legacy checkpoints.

Overview
Removes the DefaultAgentType/isSpecificAgentType backward-compat layer so agent type is no longer silently coerced to the generic "Agent" value.

resolveAgentType now prefers a non-empty SessionState.AgentType and otherwise uses the hook/context agent type, and session initialization only sets AgentType when currently unset (no more replacing "Agent" with a later value). ResolveAgentForRewind no longer falls back to a default agent; missing/unknown agent types now surface as errors, with tests updated accordingly and the legacy backfill test removed.

^{Written by Cursor Bugbot for commit 74fcb66. Configure here.}

[Copilot]

Pull request overview

This PR removes backward-compatibility fallbacks for sessions without agent type information, simplifying the codebase by eliminating dead code paths that were only needed for sessions created before agent type tracking was added (Jan 9, 2026 - 6 days after repo creation).

Changes:

• Removes DefaultAgentType constant and isSpecificAgentType() helper function that supported backward compatibility
• Simplifies agent type resolution to fail fast instead of falling back to defaults
• Removes backfill logic that replaced unknown agent types with specific types
• Updates tests to expect errors for empty/unknown agent types instead of silent fallback behavior

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
cmd/entire/cli/strategy/common.go	Removes DefaultAgentType constant and isSpecificAgentType() helper; simplifies resolveAgentType() to return context value directly without fallback
cmd/entire/cli/strategy/manual_commit_rewind.go	Simplifies ResolveAgentForRewind() to call agent.GetByAgentType() directly, removing default agent fallback logic
cmd/entire/cli/strategy/manual_commit_hooks.go	Removes backfill logic that replaced "Agent" with real agent type; simplifies InitializeSession to only set agent type when empty (not when "Agent")
cmd/entire/cli/strategy/manual_commit_test.go	Removes TestInitializeSession_BackfillsUnknownAgentType test and updates comments to remove outdated bug reference
cmd/entire/cli/strategy/rewind_test.go	Updates TestResolveAgentForRewind to expect errors for empty agent types instead of defaulting to Claude