feat: add a local Discord protocol sidecar

[Peyton-Spencer]

Summary

Adds a device-local Discord protocol sidecar as the authoritative read/write transport while retaining the existing macOS Accessibility sender as an explicit fallback.

The sidecar is built on the BSD-3-Clause github.com/beeper/discordgo fork. It keeps Discord credentials on the device, connects directly to Discord Gateway/REST, and returns Discord message IDs as authoritative send receipts. No Matrix homeserver or mautrix-discord code is embedded.

Changes

• add a local Go sidecar with QR remote-auth, reconnect/resume, message fetch, send, replies, and prepared attachment uploads
• store the Discord credential in the OS credential boundary rather than the ordinary inbox database
• package/build the sidecar for desktop development and release artifacts
• add typed desktop IPC and server adapter boundaries
• retain the fail-closed Accessibility transport for an explicit fallback path only
• document BSD dependency provenance, AGPL reference boundaries, and Discord account-policy risk
• add a dedicated Go CI job with formatting, tests, vet, and a Darwin arm64 cross-build

Send guarantees

• successful protocol sends return the created Discord message ID
• attachment sends use Discord's prepare/upload/create flow with bounded file count and size
• idempotency and target identity are explicit inputs
• ambiguous UI automation never becomes an inferred success

Validation

• Go tests, vet, and Darwin arm64 build pass locally
• focused desktop transport tests pass
• server, web, and desktop typechecks pass on the stack head
• native sidecar and Accessibility helper build as arm64 Mach-O executables
• GitHub CI is running on the corrected stacked head

Stack

Layer 2 of 6 in Stack pingdotgg#13, based on #6 (feat/discord-media-cache). Draft only; do not merge until live QR/login/send QA is complete.

Note

Add local Discord protocol sidecar and macOS Accessibility reply transport

• Adds a Go-based Discord sidecar (native/discord-sidecar/main.go) that runs as a stdio JSON-lines RPC server, supporting QR login, connection restore, conversation/message listing, and message sending with idempotency receipts and attachment uploads (10 files, 25MB/file, 100MB total limits).
• Adds a TypeScript client (DiscordSidecarClient.ts) and channel adapter (DiscordLocalAdapter.ts) on the server to spawn and communicate with the sidecar, mapping sidecar responses into the Ditto channel contract.
• Adds a macOS Accessibility helper (main.swift) and desktop transport (DiscordAccessibilityTransport.ts) that drive Discord via deep links and AXUIElement to prepare or send replies, with duplicate detection, concurrency guards, and timeout/cancel support.
• Wires up desktop IPC handlers, preload bridge, build scripts for mac packaging, and web composer UI changes to show 'Send via Discord' when server-side send is unavailable.
• Risk: the accessibility helper uses macOS Accessibility APIs and synthesized keypresses; reviewers should check performSend in main.swift and the deep-link validation in discordDeepLink in DiscordAccessibilityTransport.ts for permission and input safety. The sidecar pins discordgo to a Beeper fork commit in go.mod.

^{Macroscope summarized cdacc00.}

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

ℹ️ No successful main baseline artifact is available yet. This run establishes the initial measurement.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	—	13.3 KiB	—	15.1 KiB	✅
Codex	Thread snapshot wire	—	6.9 KiB	—	7.3 KiB	✅
Codex	Live turn WebSocket wire	—	6.4 KiB	—	7.8 KiB	✅
Codex	Live turn WebSocket decoded	—	55.6 KiB	—	66.4 KiB	✅
Codex	Live turn messages	—	11	—	21	✅
Claude	Total thread wire	—	13.3 KiB	—	15.1 KiB	✅
Claude	Thread snapshot wire	—	6.9 KiB	—	7.3 KiB	✅
Claude	Live turn WebSocket wire	—	6.5 KiB	—	7.8 KiB	✅
Claude	Live turn WebSocket decoded	—	56.4 KiB	—	66.4 KiB	✅
Claude	Live turn messages	—	11	—	21	✅

Baseline: unavailable · PR result: cdacc00 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 109.4 KiB
• Claude decoded thread snapshot: 110.1 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.