feat(discord): channel-scoped allowlist for guild messages

[BCDel89]

Summary

• Adds discord.channelAllowedUserIds config (channelId -> user IDs[]) — grants a user access to one specific guild channel without adding them to the global allowedUserIds list
• Channel-scoped access never applies to DMs, even for the same user ID — the global allowedUserIds list remains the only path to DM access
• Extracted the composed auth check into isDiscordAuthorized() in allowlist.ts so it's unit-testable independent of the Discord message-handling flow

Why

Previously allowedUserIds was all-or-nothing: a user was either allowed to reach Sage everywhere (DMs + every guild channel), or blocked everywhere. No way to grant someone scoped access to a single project channel.

Testing

• 7 new unit tests in tests/allowlist.test.ts covering: global allowlist access in DMs and guild channels, channel-scoped access in the correct channel, explicit denial in other channels, explicit denial via DM (the key leak this feature must avoid), denial for unlisted users, and no-throw on missing config
• Full suite: 117/117 passing, no regressions
• tsc --noEmit: no new errors introduced
• Manually verified live against Discord: a channel-scoped test user got real responses in their assigned channel, no response in other guild channels, and an explicit "Unauthorized." on DM — while the existing globally-allowed user was unaffected in all contexts

[TerrysPOV]

Well-built feature. I ran an adversarial fail-open/bypass audit on the auth logic and it's fail-closed on every path: isDiscordAuthorized checks the global allowlist first, gates the channel-scoped branch on isGuild, and ends with a terminal return false. Verified in particular:

• DM bypass is impossible — channel-scoped entries are isGuild-gated, so DMs (no guild_id) can only match the global allowlist. Locked by a test.
• Empty/missing/malformed-shape config fails closed (?? [] → isAllowed([]) → false).
• Wrong-channel access, type confusion, threads, and slash/button interactions are all deny-direction, and #185's front-door hardening is preserved (global-only interaction gate untouched).

Maintainer-committed the version housekeeping in 006a0d8: the branch's 1.0.41 bump conflicted with master (now 1.0.42 after #234/#247), so I merged current master in and re-bumped to 1.0.43. Please rebase on latest master and run the bump scripts yourself for future PRs.

Two non-blocking follow-ups:

1. The config.ts parser's sanitization of malformed channelAllowedUserIds (non-object, string-instead-of-array, numeric IDs) isn't unit-tested — the "malformed config can't grant access" property is only asserted for the undefined shape. The code is safe; a couple of parseSettings tests would pin it.
2. Two safe-but-undocumented asymmetries worth a doc line: channel-scoped users can send messages but not use slash-commands/buttons (interaction gate is global-only), and their access doesn't extend into threads (distinct channel_id).

Approving.