fix(agent): stop stack overflow in agent_factory test under full features

[bug-ops]

Summary

• VigilGate::try_new (crates/zeph-core/src/agent/vigil.rs) recompiled the entire 14-pattern bundled injection bank via regex::Regex::new() on every Agent construction — the only one of 7 consumers of RAW_INJECTION_PATTERNS that didn't already cache the compile behind a LazyLock. Cached it behind a process-wide static, matching the six sibling call sites (zeph-sanitizer, zeph-mcp, zeph-skills). Independent perf fix, not itself sufficient to prevent the crash (nextest runs each test in its own process, so the cache is cold on first touch regardless).
• Under --features full, one bundled pattern triggers regex_automata's SIMD Teddy-prefilter builder, whose frame overflowed the default 2 MiB test-thread stack when reached ~25 frames deep through serve::agent_factory::tests::build_agent_factory_gates_trust_state_independently_per_session's two-Agent construction chain (inflated further by full's unboxed AnyProvider::Candle/Gonka/Cocoon variants). Gave that test a dedicated 32 MiB-stack thread (named, with a resume_unwind panic passthrough), matching both MAIN_THREAD_STACK_SIZE (src/main.rs, zeph --daemon crashes with stack overflow on every invocation #5394) and the coverage job's existing RUST_MIN_STACK: "33554432" (added in 9a1efb89a for this exact test under instrumented coverage) — the explicit stack_size() would otherwise have silently overridden that CI protection down from 32 MiB to a smaller value.
• Updated the coverage job's CI comment to reflect that this test now self-manages its stack, while keeping the env var (it still protects other, unnamed multi-agent tests).

Closes #6699

Test plan

• cargo test -p zeph --bin zeph --features full serve::agent_factory::tests::build_agent_factory_gates_trust_state_independently_per_session -- --exact — passes (previously SIGABRT)
• cargo nextest run --config-file .github/nextest.toml --workspace --features "desktop,ide,server,chat,pdf,scheduler,testing" --lib --bins — 15258 passed, 0 failed
• cargo +nightly fmt --check — clean
• cargo clippy --profile ci --workspace --all-targets --features "desktop,ide,server,chat,pdf,scheduler,testing" -- -D warnings — clean
• RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps --workspace --features "desktop,ide,server,chat,pdf,scheduler" — clean
• gitleaks protect --staged — no leaks