inspect: remove dead code and collapse the synthesis path

Cleanup pass over the review/inspect feature after a long iteration.

Dead code:

• delete the unused agent multi-picker (multipicker.go) and PromptForAgent; the multi-agent path fans out over all eligible agents, no picker
• delete the unused trail.Store local-storage layer (store.go) and the ID/Priority/Type/Reviewer/Discussion/Checkpoints types it alone used
• drop TrailDetailResponse and write-request fields the CLI never sends (TrailUpdateRequest.{Branch,Base,Assignees,Priority,Type}, TrailCreateRequest.{Assignees,Labels,Priority,Type}, BranchCreated)
• delete EntireSettings.ReviewConfigFor (only its own test read it); the legacy Review/ReviewFixAgent/ReviewMigrationDismissed fields stay as parse-tolerance shims (the loader uses DisallowUnknownFields)

Collapse the synthesis path: autoSynthesis was always true in production, so the prompted (Auto=false) branch was unreachable. Drop SynthesisSink's Auto/InputTTY/PromptYN, the legacy compose branch, realPromptYN, and the now-dead canPrompt/promptYN sink inputs. The master report now runs unconditionally in TTY and redirected output alike.

Tidy: delegate top-level trail watch to the shared trail-review resolver (deleting the bespoke resolveTrailWatch* duplicates), make RunReviewProfileConfigPicker return only error, inline single-caller wrappers (detectScopeBaseRef, hydrateTrailReviewCommentSuggestions, reviewTrailFindingInput, saveDefaultReviewProfile), and fix flushBuffer's dead error-handling return.