switch_profile: deliver profile_switched, report the new toolset ...

The tool took its sink as `ctx.event_sink if ctx else current_event_sink.get()`,
but the agent loop builds tool_ctx with event_sink=None — the ContextVar is the
real channel — so the branch always picked None, the event was silently
dropped, and the profile badge in the header never moved. plan.py already had
the fall-through (`if ctx and ctx.event_sink else current_event_sink.get()`);
this is the same fix.

The result also said nothing about what the switch changed, so the model kept
calling tools the target profile does not have (reload_tools after leaving
tool_developer). It now names the gained and lost tools, and the timing claim
is corrected: the new prompt and tools are in force from the next step of the
same turn, not "from the next message".

Allocate message sequence numbers in the DB, not in memory ...

save() numbered new messages from Session.db_next_sequence, a counter read
when the session was loaded. Any second writer of the same session handed
out the numbers it still believed were free, and the two inserts collided on
UNIQUE(session_id, sequence_number) — in production, when switch_profile
loaded the session mid-run and saved it back. The turn died with
"Internal error: duplicate key value violates unique constraint".

The range is now claimed inside save()'s transaction with a single
UPDATE ... RETURNING, so concurrent writers serialize on the session row,
and GREATEST() seeds the pre-counter sessions whose next_sequence is still 0
instead of relying on a racy max()+1 fallback in memory.

switch_profile itself no longer saves a session at all: it repoints the row
through a narrow set_profile() UPDATE, which keeps it out of the running
turn's way. It runs mid-turn on a session the turn still holds, so saving a
second copy from there was the collision in the first place.