fix(api): pre-populate Agent.messages from DB on construction so model switches preserve chat history

Before this change, swapping the model mid-conversation via the sidebar
slider lost all prior turns: the new model saw only the current user
message and treated the conversation as brand-new.

Root cause: `getOrCreateAgentForTab` invalidates the cached Agent
(`tabAgent.agent = null`) whenever the effective keyId, modelId,
permission key, or working directory differs from the cached values.
The replacement Agent was then constructed with `messages: []` and
the post-construction step that loads prior turns from the SQLite
`messages` table simply did not exist. `processMessage` had already
appended the current turn's user message to the DB (line 960) before
calling `getOrCreateAgentForTab` (line 1015), so the DB held the full
context — it was just never read.

Fix: after every `new Agent(...)` in `getOrCreateAgentForTab`, call
`getMessagesForTab(tabId)`, walk backwards to the most recent user-role
row, and assign all strictly-prior rows to `tabAgent.agent.messages`.
The walk-backwards strategy correctly handles two boundary cases:
  1. Simple model switch — last DB row is the current user message;
     drop it (`Agent.run()` will push it again at line 546).
  2. Agent-mode auto-fallback retry — last DB row may be a partial
     assistant response flushed by the previous failed attempt; we
     drop both that and the current user message in one pass.
System-role rows (config-reload notices, etc.) are preserved verbatim;
`toModelMessages` already strips them before the wire payload, so this
is safe.

The fix covers every Agent-reconstruction trigger, not just the model
slider:
  - Sidebar model/key change (the reported case)
  - Permission setting change
  - Working-directory change (`processMessage` line 951)
  - dispatch.toml config-watcher reload (lines 236–237)
  - Skills directory watcher reload (lines 249–250)
  - `stopTab` after user cancellation (line 775)

If `getMessagesForTab` throws (e.g. DB locked, schema mismatch), we
swallow the error and leave `messages: []` — matching pre-fix
behaviour for that case so this commit never regresses.

Tests (+6 in `packages/api/tests/agent-manager.test.ts`, total 26):
  - pre-populates Agent.messages from DB history
  - leaves messages empty when DB has only the current turn (first msg)
  - excludes a partial assistant trail from a prior fallback attempt
  - preserves system-role rows in pre-populated history
  - survives a getMessagesForTab failure without crashing
  - reloads history on every Agent reconstruction (simulated slider
    switch from Opus to DeepSeek across two processMessage calls)

The test rig was extended with:
  - `fakeMessagesByTab` map + `setFakeMessages` helper letting tests
    inject arbitrary DB rows for the mocked `getMessagesForTab`.
  - `constructedAgents` array captured at `run()` entry (not at
    construction) so each test sees the post-pre-populate snapshot;
    the production code reassigns `agent.messages` after `new Agent()`
    returns, so capturing at construction yielded a stale empty array.
  - Pluggable `runImpl` hook for tests that want a custom event stream
    (not yet exercised; staged for the next round of agent-mode
    fallback tests).

Totals: 229 tests across 3 packages all green; typecheck clean on
core + api + frontend; biome clean across 124 files.

0 files changed, 0 insertions, 0 deletions