Merge run_registrations into runs; make identity columns write-once; stop agents calling create_run

[susheem-k]

Problem

run_registrations and runs are a 1:1 split keyed by the same run_id, and the
split exists mainly as a workaround: runs is clobberable, run_registrations is not.

• register_run (control_plane/store.py) writes both: INSERT into
  run_registrations (guarded — RunAlreadyRegisteredError / 409) and create_run
  → REPLACE INTO runs.
• Every example agent server (entry and downstream: research, summarize,
  writer, analyst, editor, triad *) also calls client.create_run(...) on
  entry (PUT /v1/run-records → REPLACE INTO runs) and client.update_run(...) at
  close.

Result for a 2-agent run sharing one run_id

step	writer of the single runs row	effect
researcher POST /v1/runs	register_run → create_run(agent=intent, dims={user_id:alice}, task=intent)	row created
researcher handler	client.create_run(agent="researcher", task=<text>, started_at=...) — no dims	REPLACE → dims wiped to {}
writer handler	client.create_run(agent="writer", parent_span=span_1, started_at=...)	REPLACE → agent="writer", started_at=writer clock
writer finally	update_run(status=completed, cost_micros, steps=<writer local>)	UPDATE
researcher finally	update_run(status=completed, cost_micros, steps=<researcher local>)	UPDATE (last wins)

Final runs row: agent="writer" (the leaf, not the entry), dims={} — user
attribution lost from runs (survives only in run_registrations), started_at =
writer's clock, steps = one process's local count. Incoherent, last-writer-wins.

Desired

One runs table:

• write-once at registration, never in the update path: run_id, intent/task,
  user_dims, mode, parent_run, parent_span, registered_at, started_at.
• mutable, only via PATCH with an explicit allow-list (update_run already takes
  **fields — enumerate them): status, cost_micros, steps, halt_reason,
  detector, ended_at, governance_events.
• keep the INSERT-or-409 guard on that one table.
• remove client.create_run(...) from every agent server. Registration creates the
  row; agents only PATCH. Downstream agents create nothing (they already do not
  register).
• resolve_run = SELECT ... WHERE run_id=?; "no row" ⇒ not registered (or a
  registered_at IS NOT NULL check).
• drop create_run from the store protocol / HttpStore (or make it registration-only).

Out of scope

Per-agent participation rows. A single top-level run row is fine — per-agent spend is
already captured in ledger_spent (add an agent-dimension budget/accumulator if a
per-agent breakdown is wanted; segment_key_for already supports dimension="agent").
steps should come from the plane's own step accounting once RunState moves server-side
(plan Part 6), not from a client's local count.

Related: #55, #60, #113, plan Part 6 / Part 9b.