fix(subagent,core): replay journaled result on durable subagent resume

[bug-ops]

Summary

• zeph-subagent's durable-promise adapter (durable.rs) implemented the "finished-child replay" case documented since feat(sub-agents): durable promise for subagent spawn/await resume (P4) #4954, but the only production call site in zeph-core discarded the resumed promise unconditionally and always re-spawned the sub-agent, duplicating LLM calls and any side-effecting tool calls (git operations, GitHub issue/PR creation, file writes) a finished child had already performed.
• maybe_make_durable_seat is replaced by resolve_durable_spawn_gate (DurableSpawnGate::{Fresh, Replayed, None}). On a resumed run it performs a non-blocking check via the new zeph_subagent::try_replay_durable_subagent (wrapping DurableContext::take_resolved_promise, now pub) for whether the child already resolved before the crash.
• handle_agent_background / handle_agent_spawn_foreground skip mgr.spawn(...) entirely when the promise already resolved, feeding the journaled SubagentResult through the normal completion-notification path instead.
• A still-pending resumed promise (child's fate unknown after a crash — resolver token unrecoverable per INV-9) falls back to the pre-existing spawn behavior, matching the documented v1 scope boundary, and now emits a tracing::warn! so the residual duplicate-spawn window is observable rather than silent. Tracked as a known v1 limitation in durable sub-agent resume: still-pending promise falls back to spawn, may duplicate a genuinely in-flight child #6010.
• specs/064-durable-execution/spec.md §888 updated to record that the v1 LocalBackend integration falls back to spawn rather than re-parking, and why.

Closes #5944.

Test plan

• cargo +nightly fmt --check
• cargo clippy --profile ci --workspace --all-targets --features "desktop,ide,server,chat,pdf,scheduler,testing" -- -D warnings
• cargo nextest run --config-file .github/nextest.toml --workspace --features "desktop,ide,server,chat,pdf,scheduler" --lib --bins (12695/12695 passing, includes 3 new end-to-end regression tests exercising handle_agent_background/handle_agent_spawn_foreground through the replay branch and the still-pending fallback, using two independently-constructed Agent instances over a shared on-disk sqlite journal to simulate a real parent-process restart)
• RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps --workspace --features "desktop,ide,server,chat,pdf,scheduler"
• cargo test --doc -p zeph-subagent -p zeph-durable -p zeph-core
• Adversarial critique (rust-critic): verdict significant → 1 gap (silent duplicate-spawn fallback path) fixed with a warning log + follow-up issue durable sub-agent resume: still-pending promise falls back to spawn, may duplicate a genuinely in-flight child #6010; core replay/fallback design explicitly approved
• Code review (rust-code-reviewer): verdict approved, all check-suite gates independently re-verified