fix(headless): persist tool calls on -p --session (resume cutoff); mermaid validation

[yogthos]

The resume "cutoff" bug

Found while testing the MCP delegation loop. dirge -p --session <id> saved the conversation, but the assistant message stored only its final text — every tool call/result was dropped (verified: a saved MCP session had tool_calls=0 on assistant turns that actually wrote/edited files). Consequences on resume:

• A resumed --session (e.g. an MCP delegation follow-up) lost the substance of prior turns — dirge saw "I did X" but not what the tools actually did.
• A tool-heavy final turn — which often ends with little or no trailing text — saved an empty/thin assistant message, reading as a cut-off end.

(The companion resume fix — feeding the loaded history back to the model at all — already shipped in #406. This adds full fidelity.)

Fix: run_print now accumulates the turn's ToolCall/ToolResult events (mirroring the interactive run_handlers/tool_call.rs + tool_result.rs) and returns them; the print path saves via add_message_with_tool_calls, so convert_history re-emits the tool_use/tool_result blocks on resume. The --loop caller ignores them.

Verified live: dirge -p --session X "create a file FOO.txt" now saves the assistant message with tool_calls=[write, read] instead of 0.

Plugin review: plugins/mermaid_diagram.janet

Reviewed as requested. The structure is sound (the stem-prefixed hook names are a supported form; request-prompt/append-system-prompt/3-arg add-custom-message all exist). Fixed validation gaps in validate-diagram:

• The edge check only matched --> / ->> / ---, so valid er/class/state diagrams (and flowchart -.-> / ==>) were rejected for "no connections". Broadened to -- / ->> / ==> / -.-.
• Added a {}-balance check ({decision} / {{hexagon}} nodes were unchecked).
• Dropped a stray [ from the unbalanced-brackets error message.
• max-retries → def (never reassigned).

Parses clean (janet parse-all).

Tests

Full suite 2612 pass; run_print signature change covered by the existing print tests + live verification.