fix(tracing): replace span.enter() across .await with .instrument()

[bug-ops]

Summary

• Fix 6 locations where a synchronous span.enter() guard was held across an .await boundary (!Send, causes incorrect trace parent-child relationships under tokio multi-thread) — replaced with .instrument(span).await or async move { ... }.instrument(span) for spawn() blocks.
• Normalize 8 span name strings in zeph-tools from the inconsistent tool.* / scrape.* prefixes to the canonical tools.<subsystem>.<operation> convention from CLAUDE.md.

Files changed

File	Change
zeph-core/src/agent/agent_access_impl.rs	.instrument(span).await on semantic scan future
zeph-core/src/agent/mod.rs	.instrument(span) on supervisor async move spawn block
zeph-context/src/fidelity.rs	.instrument(span).await on timeout(p.chat(...))
zeph-skills/src/evaluator.rs	.instrument(span) inside timeout async block
zeph-tools/src/shadow_probe.rs (×2)	.instrument(span).await on both probe calls
zeph-tools/src/{scrape,shell/mod,diagnostics,file,search_code}.rs	Span name renames

Test plan

• cargo +nightly fmt --check — clean
• cargo clippy -p zeph-core -p zeph-context -p zeph-skills -p zeph-tools -- -D warnings — clean
• cargo nextest run -p zeph-core -p zeph-context -p zeph-skills -p zeph-tools — 3579/3579 passed
• RUSTFLAGS="-D warnings" cargo check -p zeph-core -p zeph-context -p zeph-skills -p zeph-tools — clean

Notes

• The mod.rs supervisor span becomes a sibling of bg_task instead of its child (span was moved from inside the async move block to the call site). Cosmetic trace-tree change only, no behavioral impact.
• 4 additional span.enter() anti-patterns were found outside this PR's scope during review: tracked in bug(core): span.enter() across .await anti-pattern in 4 remaining locations #4787 (P2: memcot/accumulator.rs:158 has extra record() fragility; P3: 3 locations in zeph-plugins).

Closes #4777, #4714