feat: Language Threads — mutual-aid language sidecars

[daopunk]

Summary

Replaces the old per-user “translate my messages into the group” behavior with Language Threads: one Signal sidecar group per subscribed language, bridged to a bilingual main mutual-aid chat.

• Main → sidecar: same language relays; otherwise translates via NEAR AI
• Sidecar → main: relay only (main stays bilingual)
• Sidecar → other sidecars: translate
• Bot never relays its own messages (BotIdentity: phone + learned UUID)
• Sidecar naming: Language Thread {Language} (e.g. Language Thread Spanish)
• Alpha command surface slimmed: !translate-me-on / off, !list-langs, help/privacy/verify/set-lang (ask/chat/voice/translate-on kept in tree but unregistered)

Docs: docs/language-threads.md

Commits

1. f4c6c22 — language sidecar bridge (!translate-me-on)
2. e9e2ab4 — rename BAM → Language Thread + slim help
3. cac6a81 — Language Threads overview doc

Notable code

Area	Path
Subscribe + relay	crates/signal-bot/src/commands/translate_me.rs
Bot skip	crates/signal-bot/src/bot_identity.rs
Encrypted bridge store	crates/signal-bot/src/group_preferences_store.rs (LanguageBridge)
Group REST	crates/signal-client — create_group / add_members / remove_members
Envelope fields	source_name / source_number on BotMessage
Handler registration	crates/signal-bot/src/main.rs

Test plan

• cargo test -p signal-client -p signal-bot --lib
• Local Docker: rebuild signal-bot; !translate-me-on es creates/joins sidecar; invite accepted; main↔thread relay/translate works
• Phala CVM deploy (paused): push image done (daopunk/signal-bot-tee:latest linux/amd64); deploy dstack-app-hqvaf on tdx.medium (4 GB) still pending
• After TEE deploy: !help, !translate-me-on, bridge message, !verify; re-register Signal phone if volume is fresh
• Confirm missing sourceNumber UX (DM bot once, then retry invite)
• Loop check: bot-authored posts do not fan out again

Notes

• Translation uses NEAR AI (near_ai_translate); same-language traffic is relay-only.
• Bridge state persists in encrypted group prefs (TEE key on Phala; may not persist locally without dstack).
• Phala pulls Docker images, not git — public image registry matters more than branch visibility for the CVM.

[daopunk]

Product follow-ups:

• Product: split translation into in-group vs Language Threads #9 — Split translation into in-group vs Language Threads (this PR is the Language Threads product)
• Product: extract voice transcription to its own CVM (4 GB) #8 — Extract voice transcription to its own 4 GB CVM, interoperable with both translation products