Cross-reference gateDryRun and agentDryRun so they aren't assumed to be one system

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Context

gateDryRun and agentDryRun both use "dry run" vocabulary but gate two entirely disjoint layers with no shared code path. gateDryRun only computes an extra displayConclusion preview on the check-run (src/rules/advisory.ts:534-558) — the real posted conclusion is unaffected. agentDryRun switches the action-layer's execution mode to suppress real GitHub mutations (src/settings/agent-execution.ts:38-41) — entirely independent. Both fields are exposed side-by-side, unlinked, in the same maintainer-facing template config/examples/gittensory.full.yml — gate.dryRun at line 315 (inside gate:) and settings.agentDryRun at line 706 (inside settings:), with no cross-reference anywhere.

Requirements

• Minimal fix, no breaking rename. (A heavier alternative — renaming gateDryRun → gatePreviewMode, config-as-code only, no migration needed — is optional and out of scope here.)

Deliverables

• types.ts gateDryRun doc: append "Does NOT stop the agent action layer from performing real merges/closes/comments — see agentDryRun for that."
• types.ts agentDryRun doc: append "Independent of the gate check's own gateDryRun preview."
• .gittensory.yml.example and config/examples/gittensory.full.yml: add matching cross-reference comments at both sites (gate.dryRun and settings.agentDryRun).

Expected Outcome

A maintainer reading either "dry run" field's doc comment or yml example is pointed at the other one, and understands the two systems are unrelated.

Links & Resources

• Evidence: src/types.ts:795-798,1091-1093; src/rules/advisory.ts:534-558; src/settings/agent-execution.ts:38-41; config/examples/gittensory.full.yml:315,706

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Part of #5270 (roadmap epic).

[linear-code]

JSO-3783