feat(composer): park the Conversation Mode picker

[philmerrell]

Removes ConversationModePickerComponent from the composer and deletes it. The placement works — I verified it end to end on dev before pulling it — but a permanent composer slot is a bigger commitment than the evidence currently supports, so it comes out until there is user feedback to decide the location on.

Only the control is removed

Everything behind it is untouched and still live:

Layer	State
Admin CRUD /admin/system-prompts	unchanged
GET /system-prompts/ catalog	unchanged
SystemPromptsService (load, active-prompt tracking, step-5 hydration fix)	intact
selected_prompt_id on SessionPreferences	intact, still round-trips
The picker component	deleted

So restoring this is git revert on one commit, not a rebuild — and re-homing it elsewhere starts from a tested component rather than a blank file.

⚠️ Release gate — please read before the next release to main

origin/main still carries components/model-settings/. The Customize epic has not reached prod, so prod users select Guided Learning through the old drawer today — ~60 sessions in the first 12 days of September and accelerating.

develop deletes that drawer (#1079) and, after this PR, has no picker. The first release carrying both to prod therefore ships no way to select a Conversation Mode at all, silently killing a feature in active growth.

Before that release: restore the picker, land a replacement placement, or take the regression knowingly and tell @derrickfink. This is recorded in docs/specs/customize-surface.md so it surfaces at release time rather than after.

Placements still on the table

Conversation title menu · inside the model dropdown · new-chat empty state · a /mode inline command (copy agent-mention-menu.component.ts) · generalising the assistant indicator — which today renders only when an Agent is bound (topnav.html:140), so it cannot host Mode as-is.

Verification

• 2842 tests / 240 files pass; production build clean.
• The obvious browser check was worthless and I want to flag why: localhost:4200 points at a local app-api whose prompt catalog is empty, so the chip would be absent there whether or not anything changed.
• The check that actually proves it: the picker's strings return 0 matches in dist/, against a control string returning 3. The control cannot render for anyone, regardless of catalog contents.
• Two seeded modes are deliberately left enabled on dev so the feature can be re-exposed without re-seeding.

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