feat(budgets): per-task and per-project cost budgets (flag-gated) (#654)

* fix(notifications): exponential backoff + CAS claim for expired-unacked re-escalation

The sweep re-escalated every expired unacked ack-required notification
on every ~60s tick, forever — the live incident: 3 fresh blocker
escalations + Telegram DMs per minute from a static stale pile. Now
each notification carries reescalation_count / last_reescalated_at /
reescalation_delivered_count (migration 079): first fire at expiry,
then doubling intervals from 1h capped at 24h, hard stop after
ROBOCO_NOTIFICATION_MAX_REESCALATIONS (default 5) with one permanent
log carrying attempts-vs-delivered so 'seen and ignored' is
distinguishable from 'route never worked'. The due/wait/capped decision
is a pure function in foundation/policy/communications.py.

Per adversarial review, the attempt slot is claimed by compare-and-set
(UPDATE ... WHERE reescalation_count = :n) BEFORE delivery — the
previous draft leaned on the 60s dedup window, which never engages for
BLOCKER_ESCALATION (_LOOP_PRONE_TYPES excludes it), so concurrent
sweeps would have double-delivered. A lost claim skips delivery
outright. Legacy rows read as count=0 and keep today's first-fire
semantics. 61 tests incl. a two-session CAS race and a real alembic
upgrade/downgrade round trip.

* feat(budgets): per-task and per-project cost budgets (flag-gated)

tasks.budget_usd + projects.monthly_budget_usd (migration 080, chained
on 079; adds ix_agent_spawn_sessions_task_id since both enforcement
seams filter on bare task_id). Behind ROBOCO_TASK_BUDGETS_ENABLED
(default off, feature-flags card) — verifiably inert when off.

Claim-time: a project-month-spend guard applies to WORK-STARTING claims
only (i_will_work_on / i_will_plan) — per adversarial review, review/
doc/gate/inbound-PR claims are exempt so in-flight work can always
finish reviewing and merging at cap. Spend counts closed sessions'
estimated_cost_usd PLUS open sessions priced live from token snapshots
(the original closed-only sum read parallel long sessions as $0).

Sweep-side: the existing budget sweep also prices the active task's
spend vs budget_usd (TaskType defaults when null); on breach the task
is BLOCKED (HUMAN resolver, budget marker) BEFORE the graceful stop so
the unclaim no-ops and the dispatcher never respawns onto it, and the
CEO notification names both recovery steps. unblock on a budget-blocked
task re-checks live spend and refuses while still over — no silent
re-breach loop. Panel: budget inputs in both dialogs (0 rejected — a
zero budget silently blocks everything), spend logic consolidated in
TaskService.task_spend_usd. 42 new tests incl. a real-DB spend-query
suite and a two-tick non-refire sweep test.

---------

Co-authored-by: Renn F <rennf93@users.noreply.github.com>

tests/unit/config/test_task_budgets_flag.py

@@ -0,0 +1,27 @@
+"""Per-task and per-project cost budgets are gated by a default-off config
+flag, registered as a panel-tunable feature flag (mirrors possibilities_matrix)."""
+
+from __future__ import annotations
+
+import os
+from unittest import mock
+
+from roboco.config import Settings
+from roboco.services.settings import FEATURE_FLAGS, validate_setting
+
+
+def test_task_budgets_disabled_by_default() -> None:
+    assert Settings().task_budgets_enabled is False
+
+
+def test_task_budgets_reads_env_var() -> None:
+    with mock.patch.dict(os.environ, {"ROBOCO_TASK_BUDGETS_ENABLED": "true"}):
+        assert Settings().task_budgets_enabled is True
+
+
+def test_task_budgets_flag_registered_in_feature_flags() -> None:
+    assert "task_budgets_enabled" in [key for key, _ in FEATURE_FLAGS]
+
+
+def test_task_budgets_flag_validates_as_bool() -> None:
+    validate_setting("task_budgets_enabled", "true")