fix(security): redact secret fields in IbctKey(Config)/McpTransport Serialize

[bug-ops]

Summary

• Replaces the derived Serialize on IbctKeyConfig, IbctKey, and McpTransport with hand-written impls that redact their secret fields (key_hex, key_bytes, env values, headers values) to "[REDACTED]", closing the residual gap PR fix(config,mcp,a2a): redact three more secret-bearing Debug/Serialize leaks #6005 intentionally left when it fixed only Debug.
• Deserialize is untouched on all three types — config load, --init/--migrate-config, and ACP mcp/add are unaffected.
• Deletes the now-dead hex_bytes::serialize function in crates/zeph-a2a/src/ibct.rs (only deserialize is referenced once Serialize is no longer derived on IbctKey).
• ServerEntry needs no change — it nests McpTransport and inherits the redaction automatically (verified, not assumed).

A security audit confirmed this was a latent risk, not a live leak: no code path today serializes these types to JSON/logs/status output (ACP mcp/list only serializes server IDs as Vec<String>; mcp/add only deserializes). No round-trip/persistence path exists for any of the three types, so the redaction is behavior-preserving.

A same-defect-class cohort (TelegramConfig.token, DiscordConfig.token, SlackConfig.bot_token/signing_secret, A2aServerConfig.auth_token, McpServerConfig.env/headers) was identified but intentionally scoped out of this PR — those legitimately serialize to TOML via the --init wizard and need per-field assessment before a fix (blanket redaction would corrupt generated configs). Filing a follow-up tracking issue.

Closes #6006

Test plan

• 6 new regression tests (IbctKeyConfig, IbctKey, McpTransport::Stdio, McpTransport::Http) asserting the serialized output contains "[REDACTED]" and does not contain the raw secret value
• cargo +nightly fmt --check — clean
• cargo nextest run --config-file .github/nextest.toml --workspace --features "desktop,ide,server,chat,pdf,scheduler" --lib --bins — 13213 passed, 0 failed
• RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps --workspace --features "desktop,ide,server,chat,pdf,scheduler" — clean
• cargo clippy --profile ci --workspace --all-targets --features "desktop,ide,server,chat,pdf,scheduler,testing" -- -D warnings — fails, but on a pre-existing, unrelated clippy::large_futures issue in src/serve/handlers.rs/src/serve/agent_factory.rs. Confirmed via git stash + clippy against plain origin/main HEAD that the failure predates this branch; this PR touches neither file. Will file separately.