feat: AI chat phase 5 (saved-query chips, user-defined slash commands)

[datlechin]

Summary

Closes the remaining items in the AI Chat umbrella (#1047):

• 5a — Saved-query chips. Type @ and pick a saved SQL query. The chip carries the favorite's name; on send the chip resolves to the query body wrapped in a SQL code block under a ## Saved Query: <name> header.
• 5b — User-defined slash commands. Author your own commands in Settings -> AI -> Custom Slash Commands. Templates support {{query}} (current editor query), {{schema}} (active schema), {{database}} (active database name), and {{body}} (text typed after the command in the composer).

Changes

5a — Saved query chips

• ContextItem.savedQuery(id:name:) carries the name alongside the id (Codable migration: missing name decodes as empty string).
• AIChatViewModel.cachedSavedQueries: [UUID: SQLFavorite] populated on attach + on connection load.
• primeAttachmentData now handles .savedQuery by looking up the favorite via SQLFavoriteManager.shared.fetchFavorite(id:).
• resolveSavedQueryAttachment(id:fallbackName:) formats the cached favorite as ## Saved Query: <name>\n\``sql\n\n````.
• loadSavedQueries() populates a per-connection list. The chat panel's .task(id: connection.id) invokes it on open and on connection change.
• mentionCandidates(forQuery:) includes saved queries (filtered by name, alphabetical) sharing the popover budget with tables (each gets up to half of the 10-slot cap).

5b — User-defined slash commands

• CustomSlashCommand model (Codable, Identifiable): id, name, description, promptTemplate.
• CustomSlashCommandStorage is @Observable @MainActor, persists to UserDefaults under ai.customSlashCommands.v1, and exposes add / update / delete / command(named:).
• CustomSlashCommandRenderer does mechanical placeholder substitution — {{query}}, {{schema}}, {{database}}, {{body}}. Unknown placeholders pass through unchanged.
• AIChatViewModel.runCustomSlashCommand(_:body:) mirrors the built-in slash command flow: appends an invocation ChatTurn, renders the template, sends through sendWithContext.
• The composer's slash menu now lists built-ins, then a Custom section with the user's commands. Items hide when command.isValid is false (empty name or template).
• Settings: CustomSlashCommandsSection slots into AISettingsView between the existing context section and privacy section. List rows show /<name> · description with Edit and Delete actions; an editor sheet handles add/edit with name, description, and a multi-line template field. Footer documents the available variables.

Schema as a substitution variable

{{schema}} is currently passed nil in runCustomSlashCommand because rendering the schema synchronously is awkward (it requires the column/foreign-key data we fetch on demand). For v1, users who want schema in their template should attach the schema chip via @Schema and reference it in the message naturally. A followup can wire ensureSchemaLoaded + AISchemaContext.buildSchemaSection into the renderer if that turns out to be a common pattern.

Risk

Low. New surface area is additive — no existing flows changed shape.

• ContextItem.savedQuery enum case shape changed (UUID → (UUID, String)). Decoder defaults name to empty for older payloads, which then resolves through the chip's fallbackName path. No persisted-data breakage.
• Custom commands persist to a fresh UserDefaults key; no migration needed.

CHANGELOG

Two new [Unreleased] / Added lines covering 5a and 5b.

Lint

swiftlint lint --strict clean across all 817 files.

Test plan

• Save 2-3 favorites for a connection. Open chat, type @, pick one. Chip shows the favorite's name. Send. AI receives the SQL.
• Open Settings -> AI -> Custom Slash Commands. Click Add. Name review, description SQL injection check, template Review this query for SQL injection: {{query}}. Save.
• In chat with editor query open, click the slash menu, pick /review. Confirm AI receives the rendered prompt with the editor query substituted.
• Edit the command from settings; confirm slash menu picks up the change.
• Delete the command; confirm it disappears from the slash menu.
• Restart the app; confirm both saved chips (when re-attached) and custom commands persist.

Umbrella status

This closes the implementation scope of #1047. Remaining followups (separate issues): write-side tools with safe-mode flow, Apple Intelligence transport (#1048), token cost surfacing.

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