fix(core): redact base64 blobs and message history from debug dumps

[bug-ops]

Summary

• dump_tool_output, dump_response, dump_tool_error, and dump_focus_knowledge in crates/zeph-core/src/debug_dump/mod.rs wrote raw text to disk with no redaction, so a tool returning binary data as plain text (e.g. a vision/screenshot tool emitting base64 image bytes instead of a typed MessagePart::Image) leaked full payloads into debug dump files.
• Added redact_binary_blobs to crates/zeph-core/src/redact.rs: replaces contiguous runs of 200+ base64-alphabet characters with a <redacted possible binary data: N bytes, blake3:...> marker (decode-then-hash, with an undecodable-run fallback). Applied alongside the existing scrub_content across all four text-dump sinks.
• dump_request (json_dump/raw_dump) also recursively redacts every string leaf of its assembled JSON payload via a new redact_json_value helper — without this, a blob closed off in dump_tool_output would resurface unredacted one turn later as message history serialized into the next request dump (found during adversarial review, same class of gap as the prior debug_dump writes raw MessagePart::Image base64 bytes to disk #6306 cycle).
• Unconditional, no new config flag — debug dumps remain an opt-in, off-by-default diagnostic aid ([debug] enabled).

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 (targeted redact/debug_dump filter: 95/95 passed)
• RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps --workspace --features "desktop,ide,server,chat,pdf,scheduler"
• gitleaks protect --staged — clean
• New unit tests for redact_binary_blobs (redaction, marker stability, short-string passthrough, non-over-redaction on IDs/hashes, undecodable fallback, proptest never-panics)
• New end-to-end tests proving each of the four dumper methods (plus dump_request) redacts through the real public call path, asserting raw-payload absence, not just marker presence
• Live-test the base64 heuristic against real web_scrape tool output for false positives (flagged during review as a recommended follow-up, not required to close this issue)

Closes #6315