feat(telegram): MarkdownV2 + summarized streaming + approval re-keying

[finedesignz]

Builds on #189. Three deliverables for the hub Telegram bridge in one PR (cohesive, interdependent — streaming uses the new MarkdownV2 helpers).

1. MarkdownV2 formatting

• New sendMessageMd / editMessageTextMd in client.ts: send with parse_mode: MarkdownV2, and on a 400 retry once as plain text so an unbalanced reserved char never silently drops a session reply (editMessageTextMd also treats message is not modified as a benign success).
• Reuses the existing escapeMarkdownV2 (full reserved set _ * [ ] ( ) ~ \ > # + - = | { } . !+`). Callers escape dynamic content, keep their own *bold* / code.
• Outbound final messages, the working/streaming message, and the inline Approve/Deny prompt all go MarkdownV2 with the fallback. Inline keyboard unaffected.

2. Summarized streaming

• New outbound-only event bus events/session-activity-events.ts (tool_use one-liners), emitted from the existing tool_use spot in ws/agent.ts. Not a dispatch path — hub/src/dispatch/ and the non-bypassable cost-cap gate are untouched.
• Bridge keeps one editable ⏳ Working… message per (chat, session): each tool_use appends a collapsed one-liner (🔧 Edit hub/src/foo.ts, throttled ~900ms, last-12 cap); thinking/text_delta dropped. A typing chat-action fires immediately then ~every 4s. On assistant_message:final the bridge stops typing and edits the same message to the full answer (>4096 chars → follow-up send).
• Reversible behind config.telegram.summarizedStreaming (TELEGRAM_SUMMARIZED_STREAMING=false → single final-blob send). Finalization + MarkdownV2 work regardless.

3. MED fast-follow from the #189 security review

• approvals.ts registry was keyed by requestId alone → when two users share one default session the bridge fanned the prompt to both and the second rememberPendingPrompt clobbered the first, locking out a valid approver with "Not allowed" (fail-closed but wrong).
• Re-keyed by (sessionId, requestId) holding a map of every authorized userId → {chatId, messageId}. takePendingPrompt(requestId, userId) folds the authorization check into the take and resolves the whole entry exactly once for whichever authorized user taps first.

QC

• bun run check-baseline GREEN: pass=956 skip=129 fail=0 (within tolerance, fail_max=0).
• New/updated tests: escaper reserved-char coverage (existing), 400→plaintext fallback (telegram-client-fallback.test.ts, cache-busted import to avoid cross-file Bun mock.module pollution), approval re-keying incl. the two-users-same-default-session case and cross-session isolation. All 60 telegram tests pass isolated and combined.
• docs/telegram-bridge.md updated in the same commit. No routes changed → no OpenAPI/docs:sync delta.

Constraints honored

• Cost cap non-bypassable; no parallel dispatch path (activity bus is read-only fanout).
• telegram-webhook.ts raw-body-before-parse + mount order untouched (mount-order.test.ts passes).
• schema.sql not touched. Smallest-diff, surgical.

Prod-code diff ~390 LOC; slightly over the 250 split threshold but kept as one PR since the three pieces are interdependent (streaming consumes the MarkdownV2 helpers).

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