docs(config): document 4 real gate knobs missing from .gittensory.yml.example

[JSONbored]

Parent: #1936

Problem

gate.size.mode, gate.dryRun, gate.selfAuthoredLinkedIssue, and gate.aiReview.closeConfidence are all fully parsed by parseGateConfig and applied by resolveEffectiveSettings — they are real, working, enforced knobs — but none of the four appear in .gittensory.yml.example's documented gate: block. Three of the four also have no DB column or dashboard UI equivalent (config-as-code only). A self-host maintainer reading the example file (the primary documentation surface for .gittensory.yml) has no discovery path for knobs that already fully work.

Requirements

• Add all 4 keys to .gittensory.yml.example's gate: block, following the exact doc-comment convention already used for every other gate sub-field (purpose, accepted values, default, one-line rationale).
• For the 3 config-as-code-only knobs (no DB column), add an explicit "config-as-code only, this cannot be set from the dashboard" note, matching the pattern aiReviewCloseConfidence already uses where applicable.
• No code changes required — this is a documentation-completeness fix for already-shipped, already-tested functionality.

Deliverables

• Updated .gittensory.yml.example with all 4 knobs documented in the gate: block.

Acceptance criteria

• All 4 fields appear in .gittensory.yml.example with accurate accepted-values, default, and scope (config-as-code-only vs. DB-backed) documentation.
• Documentation matches actual parser behavior in src/signals/focus-manifest.ts exactly (no drift between doc and code).

Expected outcome

A self-host maintainer reading .gittensory.yml.example can discover and correctly configure every gate knob this repo actually enforces, closing a silent documentation gap on already-shipped functionality.

[linear-code]

JSO-2495