feat: execution-limit admission (#1773-#1786)

[BinaryBourbon]

Combines the reviewed and approved execution-limit admission stack — #1773 through #1786, minus the host-ceiling work — into one PR against main.

What this is

Execution limits (wall_time_seconds, max_model_turns, max_estimated_cost_usd) get a typed policy resolver, versioned per-conversation storage, owner-scoped write operations, and a refusal at every door that could otherwise promise a bounded execution it cannot deliver.

Nothing is enforced yet, and that is the point. Every control is refused end-to-end via require_controls(limits, []), so a saved allowance is always %{} today. The stack makes it impossible to accept a limit the runtime cannot honour, ahead of the runtime work that will honour them.

What it contains

PR	Change
#1773	Fountain.Conversations.ExecutionLimits — the pure resolver: validate, inherit ceilings, narrow, refuse widening
#1774	Refuse requested limits at launch, attach and channel mutation, before any write
#1775	execution_allowances — versioned storage, opaque revision, optimistic locking
#1776	Check the saved allowance inside the turn transaction, under row locks
#1777	Refuse before wake probes a provider or starts a worker; interrupt stays reachable
#1778	Refuse before a prompt supersedes autonomous work on a live server
#1779	Same for the queued initial-prompt cast
#1780	Refuse before a channel resume replaces caller tools (native, OpenAI, AG-UI)
#1781	Close the boot-recovery bypass in Rehydrator.spawn_server/1
#1782	A saved JSON null is corrupt, not an unrestricted allowance to narrow
#1783	Conversations.narrow_execution_allowance/4 — owner-scoped, audited
#1784	Typed operator-owned account ceiling storage on users
#1785	Resolve launch requests against the current account ceiling
#1786	Conversations.create_execution_allowance/4 — owner-scoped, audited

Two new migrations: 20260909090000_create_execution_allowances, 20260910020000_add_account_execution_limits.

What it deliberately excludes

#1787 (host ceilings / FOUNTAIN_EXECUTION_LIMITS) is left out — it has changes requested for a missing .env.example entry. #1788 (the two-stack join) is made moot by this PR. #1789 through #1791 (allowance persistence at launch, rotation) also have changes requested, mostly about row locks taken inside the global fleet advisory lock.

How the merge was built

Three merges on top of the #1786 chain: #1782, then the #1784 -> #1774 -> #1785 chain, then current main.

Only one textual conflict, in fallback_controller.ex — resolved by keeping the execution_limits_widen clause, plus dropping a duplicate ExecutionLimits alias in conversations.ex. Both match the resolution CI already verified on #1788.

The result was checked structurally rather than by eye: the merged tree equals #1788's CI-verified tree minus #1787's commit plus #1782's commit, with both deltas fully accounted for and nothing else. All eight guard call sites survive the main merge intact.

#1774's commit appears once, not twice — 71ce96c8 and d4ea7e76 have identical patch-ids, and the copy on the #1785 chain is the one included.

Validation

Run locally on the merged tree, on a dedicated database:

• mix compile --warnings-as-errors — clean
• mix format --check-formatted at the root and in apps/fountain — clean, exit 0
• mix credo --strict — 769 files, 6963 mods/funs, no issues
• mix deps.unlock --unused — clean
• Both migrations apply from empty
• 191 tests across the ten execution-limit suites — 0 failures
• 280 tests across the suites main's two new commits touched (conversations_start, conversations_context, changeset, conversation_controller, conversations_wake, conversation_server_acp) — 0 failures
• The dedicated database was empty afterward, confirming the unboxed_run race fixtures clean up their committed rows

Closes #1773
Closes #1774
Closes #1775
Closes #1776
Closes #1777
Closes #1778
Closes #1779
Closes #1780
Closes #1781
Closes #1782
Closes #1783
Closes #1784
Closes #1785
Closes #1786

🤖 Generated with Claude Code

https://claude.ai/code/session_01EzUPkXKdmnZYeJ1sqjpnp9