Support identity-backed Slack mentions

[sentry-junior]

Junior cannot reliably turn a person reference—name, email, or GitHub username—into a real Slack mention. The durable identity work in #733 provides a foundation, but mention resolution and rendering still need an explicit product/runtime contract.

Gaps

• Model people separately from provider identities: one canonical user may have Slack, GitHub, and email identities; Slack IDs must be scoped by workspace and provider IDs must have uniqueness constraints.
• Define trusted association paths. Slack profile email can seed a match, but GitHub email is often private or non-unique; support explicit/verified linking and avoid silently merging users on weak evidence.
• Ingest and refresh Slack profile state (user ID, email, display name, deleted/deactivated status) and GitHub username/account ID. Preserve aliases so renames do not break resolution.
• Expose a narrow resolver to the agent: lookup by Slack ID, email, display name, or GitHub username; return canonical identity plus the workspace-specific Slack user ID. Ambiguous or missing matches must fail with candidates/a clarification, not guess.
• Render mentions as Slack mrkdwn <@SLACK_USER_ID> at delivery time. Do not rely on the model to synthesize IDs or plain-text @name strings.
• Add authorization/privacy rules around exposing email and cross-provider identity data; resolver results should return only what the task needs.
• Define behavior for guests, bots, deleted users, users outside the current workspace, and multiple Slack workspaces.
• Add observability and tests covering exact match, ambiguity, renamed accounts, stale/deactivated identities, cross-workspace collisions, GitHub-to-Slack lookup, and output escaping.

Related

• Add proper identity schema to database #733 — durable identity schema/database foundation

Requested by David Cramer.

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