parity(subagent): no cumulative session-wide cap on total subagent spawns

[bug-ops]

Description

Zeph's zeph-subagent crate has two existing runaway-spawn guardrails: max_spawn_depth (recursion-depth guard, SubAgentError::MaxDepthExceeded, crates/zeph-subagent/src/manager/spawn.rs:617-620) and max_concurrent (in-flight concurrency guard, SubAgentError::ConcurrencyLimit, crates/zeph-subagent/src/manager/spawn.rs:641-644/:1157-1160). Neither bounds the cumulative total number of subagents spawned over a session's lifetime — a session could sequentially spawn an unbounded number of subagents (shallow depth, low concurrency, high total count) with no default protection, e.g. a misbehaving orchestration loop spawning-and-completing subagents in a tight sequential cycle.

This is the subagent-spawning analog of #6469 ("core: no default-on runaway tool-call / cost protection"), which found Zeph's tool-call rate limiter and daily cost budget both shipped disabled/unlimited by default, corroborated by Claude Code's session-wide CLAUDE_CODE_MAX_WEB_SEARCHES_PER_SESSION cap. #6469 was fixed by PR #6528 (rate limiting + cost cap now enabled by default).

This cycle found a second, structurally distinct guardrail from the same Claude Code release wave (v2.1.207-212, week of 2026-07-13): CLAUDE_CODE_MAX_SUBAGENTS_PER_SESSION (default 200, session-wide absolute cap on total subagent spawns, "to stop runaway delegation loops"). Zeph has no equivalent cumulative counter or config field.

Reproduction Steps

1. rg -n "max_spawn_depth|max_concurrent|ConcurrencyLimit|MaxDepthExceeded" crates/zeph-subagent/src — only depth and concurrency guards exist.
2. Inspect SubAgentConfig (crates/zeph-config/src/agent.rs) — no max_spawns_per_session-style field.
3. Observe: a session that spawns, awaits completion, then spawns again in a loop is unbounded by either existing guard.

Expected Behavior

A session should have a configurable, default-on cap on the cumulative number of subagents spawned over its lifetime, independent of depth and concurrency, to stop runaway sequential delegation loops.

Actual Behavior

No cumulative session-wide spawn cap exists; only depth and concurrency are bounded.

Environment

• Version: HEAD c63e321
• Config: default SubAgentConfig
• Features: n/a (config-level)

Evidence / Competitive Corroboration

• Claude Code (Anthropic, v2.1.207-212, July 2026): CLAUDE_CODE_MAX_SUBAGENTS_PER_SESSION, default 200, on by default, "to stop runaway delegation loops." Single reference project so far (P3 per parity severity rules; corroborating 2nd project not yet found).
• Related: feat(subagent): add recursion depth guard to prevent runaway nested spawns (GAP-06) #2579 (depth guard, closed), core: no default-on runaway tool-call / cost protection — both safety nets ship disabled #6469 (tool-call/cost runaway guardrails, closed via PR feat(core): enable runaway tool-call rate limiting and daily cost cap by default #6528) — same "default runaway protection" family, next uncovered guardrail.

Spec

.local/specs/077-subagent-session-spawn-cap/spec.md