refactor: split oversized Go files for code clarity

[Patel230]

Summary

Pure mechanical, behavior-preserving refactor for code clarity: every Go file over 1000 LOC (including test files) is split into smaller, same-package files. No behavior, API, or exported-symbol changes.

How the split was done:

• Whole top-level declarations were relocated verbatim (sliced at declaration boundaries, preserving comments and formatting).
• goimports recomputed per-file imports; gofumpt formatted.
• Build constraints (//go:build integration) and package clauses are replicated into each new file.
• One file split per commit, Conventional Commits style (47 commits).

Result:

• 47 oversized files split into 81 new same-package files (47 originals modified).
• No remaining Go file exceeds 1000 LOC (largest file in the module is now 997 LOC, pre-existing and untouched).

Verification

Check	Result
go build ./...	pass
go vet ./...	pass
go test ./...	pass (60 packages ok)

Notes:

• cli/integration_test (-tags integration) does not compile at baseline (main) in module-isolated mode (GOWORK=off) due to pre-existing references to symbols absent from this module (session.KindAgentInvestigate, strategy.SessionState.InvestigateRunID/InvestigateTopic, checkpoint.CommittedMetadata.InvestigateTopic) in investigate_test.go — a file not touched by this PR. The compile-error set is byte-for-byte identical to origin/main, and the split files in that package parse cleanly. This is a pre-existing condition, unrelated to this refactor.
• cli/api has a flaky network test (TestClient_Post_NilBody) that intermittently fails with a transport CloseIdleConnections error; it passes on re-run and is in a package not touched by this PR.

Files split (per package)

• redact (1): redact_test.go
• cli (17): explain_test.go, explain.go, setup_test.go, setup.go, status_test.go, migrate_test.go, clean_test.go, migrate.go, rewind.go, setup_github_test.go, search_tui_test.go, attach_test.go, resume.go, lifecycle.go, sessions_test.go, search_tui.go, trail_cmd.go
• cli/strategy (13): manual_commit_test.go, manual_commit_hooks.go, phase_postcommit_test.go, manual_commit_condensation.go, common.go, manual_commit_attribution_test.go, common_test.go, push_common_test.go, hooks_test.go, content_overlap_test.go, manual_commit_rewind.go, checkpoint_remote_test.go, metadata_reconcile_test.go
• cli/checkpoint (4): checkpoint_test.go, committed.go, temporary.go, v2_store_test.go
• cli/integration_test (7, build-tagged): testenv.go, manual_commit_workflow_test.go, hooks.go, agent_test.go, deferred_finalization_test.go, resume_test.go, explain_test.go
• cli/settings (1): settings.go
• cli/agent/factoryaidroid (1): transcript_test.go
• cli/investigate (1): cmd.go
• cli/summarize (1): summarize_test.go
• cli/review (1): cmd_test.go

Test plan

• go build ./...
• go vet ./...
• go test ./...
• CI green on the PR