strategy, trailers, clusterdiscovery: delete dead code and its tests

Everything here is unreachable from the CLI entry points (verified with x/tools deadcode + reference grep) and exercised only by tests dedicated to it:

• strategy/common.go: the checkpoint-lite decode cluster (ReadCheckpointMetadata, ReadCheckpointMetadataFromSubtree, decodeCheckpointInfo, decodeSessionMetadataLite, decodeSummaryLiteFromTree, plus the private lite types) along with the bench file that was its only consumer; IsInsideWorktree and GetMainRepoRoot.
• strategy: extractUserPrompts/extractUserPromptsFromLines (prod uses splitPromptContent/readPromptsFromFilesystem), subtractFiles.
• trailers: ParseBaseCommit, ParseAllSessions, FormatTaskMetadata (prod uses FormatShadowTaskCommit) and the now-orphaned baseCommitTrailerRegex. FormatMetadata/FormatCheckpoint stay — live test infrastructure.
• clusterdiscovery: exported RenderLoginHint wrapper (the private renderLoginHint stays, used by resolve.go); reword the auth/refresh.go comment that pointed at it.

Test helpers still exercising live code (hasCheckpointURL, IsAncestorOf, ManagedGitHookNames, sessionStateFile, tokenstore fault seams, OPF config seams, contexts.Save) are kept.