#8648: fix: don't treat bare numeric Discord IDs as definitive target IDs

## Summary - Bare numeric Discord IDs are ambiguous (could be user or channel) - Previously `looksLikeDiscordTargetId()` returned true for these, causing the resolver to skip directory lookup - This caused "Unknown Channel" errors when users tried to send DMs using bare numeric user IDs ## Changes - Modified `looksLikeDiscordTargetId()` to return false for bare numeric IDs - This allows the target resolver to attempt directory lookup or surface a helpful error message - Explicit prefixed targets (`user:ID`, `channel:ID`, `<@ID>`) continue to work as before ## Test plan - [x] Run type checking (`npm run check`) - [x] Run Discord target tests (`vitest run src/discord/targets.test.ts`) - [x] Run target resolver tests (`vitest run src/infra/outbound/target-resolver`) 🤖 Generated with [Claude Code](https://claude.com/claude-code) <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR adjusts Discord target normalization heuristics so bare numeric IDs (e.g. `"123"`) are no longer treated as definitive Discord target IDs. Previously `looksLikeDiscordTargetId()` returned true for `/^\d{6,}$/`, which caused upstream resolution logic to skip directory lookup and could lead to “Unknown Channel” when users intended a DM to a user ID. Mentions (`<@id>`) and explicit prefixes (`user:`, `channel:`, `discord:`) remain recognized as definitive IDs. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - Change is narrowly scoped to a single heuristic function, aligns with existing parsing behavior (bare numerics are ambiguous), and does not affect explicit target formats; no additional side effects observed in related resolver code. - No files require special attention <!-- greptile_other_comments_section --> <!-- /greptile_comment -->