Add feishu markdown card support

[luikore]

Summary

• Problem: Lark/Feishu channel sent responses as plain msg_type: "text" messages, losing all markdown formatting. Existing PR became stale and conflicts with current master.
• Why it matters: Feishu interactive cards render markdown natively (bold, code blocks, lists, links, tables), producing significantly better readability for agent responses. Approval buttons let operators approve/deny tool execution directly from the chat.

What changed:

• Markdown format feishu cards (schema v2) and tests
• What did not change (scope boundary): No changes to the Channel trait interface, config schema, other channels, or security policy.

Label Snapshot (required)

• Risk label: risk: medium
• Size label: size: S
• Scope labels: channel
• Module labels: channel: lark
• Contributor tier label: (auto-managed)

Change Metadata

• Change type: feature
• Primary scope: channel

Supersede Attribution (required when Supersedes # is used)

N/A

Validation Evidence (required)

Commands and result summary:

cargo fmt --all -- --check
cargo clippy --all-targets -- -D warnings
cargo test

• output brief:

    Finished `test` profile [unoptimized + debuginfo] target(s) in 1m 49s
     Running unittests src/lib.rs (target/debug/deps/zeroclaw-6e459f9d5a5102f5)

running 4070 tests

test result: ok. 4068 passed; 0 failed; 2 ignored; 0 measured; 0 filtered out; finished in 4.28s

     Running unittests src/main.rs (target/debug/deps/zeroclaw-e28b6c1d87f22722)

running 4065 tests

test result: ok. 4063 passed; 0 failed; 2 ignored; 0 measured; 0 filtered out; finished in 3.98s

     Running tests/test_component.rs (target/debug/deps/component-cfbc895ada0b5064)

running 176 tests

test result: ok. 176 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.10s

     Running tests/test_integration.rs (target/debug/deps/integration-b7964c933aec38a5)

running 119 tests

test result: ok. 119 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.12s

     Running tests/test_live.rs (target/debug/deps/live-9422b9bd020349ed)

running 5 tests

test result: ok. 0 passed; 0 failed; 5 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Running tests/test_system.rs (target/debug/deps/system-6a113649d3c208b8)

running 5 tests

test result: ok. 5 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.03s

   Doc-tests zeroclaw

running 3 tests
test src/providers/telnyx.rs - providers::telnyx::TelnyxProvider (line 29) ... ignored
test src/util.rs - util::truncate_with_ellipsis (line 19) ... ignored
test src/tools/schema.rs - tools::schema (line 18) ... ok

test result: ok. 1 passed; 0 failed; 2 ignored; 0 measured; 0 filtered out; finished in 0.77s

• If any command is intentionally skipped, explain why:

Security Impact (required)

• New permissions/capabilities? No
• New external network calls? No (uses existing Lark send message API)
• Secrets/tokens handling changed? No
• File system access scope changed? No

Privacy and Data Hygiene (required)

• Data-hygiene status: pass
• Redaction/anonymization notes: N/A
• Neutral wording confirmation: all test fixtures use neutral project-scoped labels

Compatibility / Migration

• Backward compatible? Yes — outgoing messages change format from plain text to interactive card; no config or API contract changes required
• Config/env changes? No
• Migration needed? No

i18n Follow-Through (required when docs or user-facing wording changes)

• i18n follow-through triggered? No

Human Verification (required)

• Verified scenarios: markdown card rendering with code blocks, bold, lists; long-message chunking across 28 KB boundary;
• Edge cases checked: multibyte UTF-8 at chunk boundary, empty content, single-chunk passthrough, exactly-at-limit content

Side Effects / Blast Radius (required)

• Affected subsystems/workflows: Lark/Feishu channel outbound message rendering
• Potential unintended effects: existing Lark users see card-formatted responses instead of plain text (visual change, no data loss)
• Guardrails/monitoring for early detection: existing health check unchanged; send failure path already logs and returns errors

Rollback Plan (required)

• Fast rollback command/path: git revert — reverts to plain text msg_type: "text" sends
• Feature flags or config toggles: none (could add a lark.card_mode config key if gradual rollout needed)
• Observable failure symptoms: Lark send errors in logs; messages not appearing in Feishu chats

Risks and Mitigations

List real risks in this PR (or write None).

• Risk:
  • Mitigation:

[theonlyhennygod]

LGTM. Well-implemented switch from plain text to interactive card messages with markdown rendering. Chunking logic correctly handles line boundaries and UTF-8 char boundaries. Good test coverage.