llm: drop model/agent scope matching; add instance-wide compaction model picker

Remove the scope system end-to-end (llm_models.scope column, agent meta
scope field, scope-based tier in model selection, UI checkboxes/pills):
it was only a soft ranking hint, had drifted (6 UI scopes vs 3 used by
agents, 'general' not even selectable) and duplicated what strength
already decides. Strength stays the single AUTO-selection axis.

Compaction: the summary model is now pickable from the Settings page
via a new PropertyType::LlmModel config property (registry key
compaction_model), instance-wide and live (no restart). Fallback chain:
explicit pick -> compaction.strength from config.yml -> priority order;
a deleted configured model degrades to AUTO. ContextCompactor reads the
key at compact time through GlobalConfigManager.

web/i18n/en.js

@@ -177,11 +177,14 @@ export default {
   'config.error_save':'Error saving "{name}": {msg}',
   'config.enabled':  'Enabled',
   'config.disabled': 'Disabled',
+  'config.llm_model.auto': 'auto',
 
   'config.set.interface.name':     'Interface',
   'config.set.interface.desc':     'Look and feel of the web interface.',
   'config.set.tic_agent.name':     'TIC Agent',
   'config.set.tic_agent.desc':     'TIC is a background agent that monitors all async events generated by connected MCP servers (new emails, calendar updates, WhatsApp messages, etc.). It reads your notification rules from data/notifications.md and your memory to decide — via an LLM call — which events are worth surfacing. Relevant notifications are forwarded to the home agent set via /sethome.',
+  'config.set.compaction.name':    'Compaction',
+  'config.set.compaction.desc':    'When a conversation grows too large, older messages are summarised by an LLM to keep the context within limits.',
 
   'config.prop.ui_locale.name':             'Language',
   'config.prop.ui_locale.desc':             'Default interface language for the whole instance. Each user can override it on their profile.',
@@ -191,6 +194,8 @@ export default {
   'config.prop.tic__security_group.desc':   'Tool permission group applied to each TIC agent session. Leave empty to use the default group.',
   'config.prop.tic__interval_minutes.name': 'Check Interval (minutes)',
   'config.prop.tic__interval_minutes.desc': 'How often TIC runs, in minutes. Leave empty to use the value from config.yml (tic.interval_secs).',
+  'config.prop.compaction_model.name':      'Compaction model',
+  'config.prop.compaction_model.desc':      'Model used to summarise compacted conversations, for the whole instance. A cheap model is usually enough. Leave empty for automatic selection.',
 
   // ── Projects ────────────────────────────────────────────────────────────────
   'projects.title':                 'Projects',
@@ -444,7 +449,6 @@ export default {
   'models.reasoning.thinking': 'Reasoning (thinking)',
   'models.strength':           'Strength',
   'models.priority':           'Priority',
-  'models.scope':              'Scope',
   'models.default_model':      'Default model',
   'models.extra_params':       'Extra params',
   'models.extra_params_hint':  '(JSON, optional)',
@@ -743,7 +747,6 @@ export default {
   'agents.detail.meta':        'Metadata',
   'agents.detail.id':          'ID',
   'agents.detail.strength':    'Strength',
-  'agents.detail.scope':       'Scope',
   'agents.detail.pinned_model': 'Pinned model',
   'agents.detail.memory_files': 'Memory files',
   'agents.detail.model_order':  'Model resolution order',
@@ -756,7 +759,6 @@ export default {
   'agents.table.strength':     'Strength',
   'agents.table.name':         'Name',
   'agents.table.model_id':     'Model ID',
-  'agents.table.scope':        'Scope',
 
   'agents.banner.title':       '<strong>Read-only view.</strong> Agents are defined by files in <code>agents/</code> — to add, remove, or modify an agent, edit the corresponding <code>AGENT.md</code> file in that directory.',
   'agents.banner.text':        'You can also ask <strong>Copilot</strong> (top bar) to create a new agent for you — just describe what it should do and it will set up all the files automatically.',