bug(tui): SSE/event pipeline defects — assistant text fallback, cancel wedge, Esc cancel, cost accumulation, /init dead code

[dennisonbertram]

Found by a live tmux-driven TUI test sweep (fake provider rigs). Evidence: .tmp/tui-test/results/ (core.md, info.md, destructive.md, inputux.md).

Bugs (acceptance criteria — each fixed with a regression test)

• Assistant replies never render for non-streaming providers: TUI only handles assistant.message.delta (cmd/harnesscli/tui/model.go:4237); the final assistant.message has no handler. Blast radius: /export missing replies, /search can't find reply text, ctrl+s copies empty text. Fix: render final assistant.message content when no deltas accumulated.
• /cancel wedges the TUI in a permanent (esc to interrupt) spinner: run.cancelled not treated as terminal in SSEDoneMsg (model.go:4445); reconnect loop subscribes to a dead run forever.
• Esc "interrupt" is cosmetic: closes only the local SSE bridge, never POSTs /v1/runs/{id}/cancel; server run executes to completion (model.go:2829 region).
• Cost/token displays regress across runs: per-run usage.delta cumulative values are assigned, not accumulated (model.go:4386-4393); status bar cost went DOWN across prompts; /cost and /context show only the latest run.
• /stats total cost shows $0.00 after spend: upsertTodayDataPoint replaces instead of adds (model.go:5071); $%.2f hides sub-cent amounts.
• /init is dead code: completeInitAgentsMd() only runs from the RunCompletedMsg handler (model.go:3910), but no production code emits RunCompletedMsg; wire pendingInitAgentsMd into the real SSEDoneMsg completion path (bridge.go:301).
• /compact transcript block vanishes at run completion; ctrl+o then expands the wrong card.
• CPR response (43;1R) leaks into the input after undo-triggered transcript rebuild; rebuild stalls ~4-5s.

Test plan

Failing Go regression tests first per bug (pattern-match sse_events_test.go, compact_command_test.go, undo_command_test.go), then minimal fixes; scripts/test-regression.sh green.

Rollout/rollback

TUI-only changes in cmd/harnesscli/tui/; rollback by reverting the branch.