feat: support {{ var | default xxx }} fallbacks in template fields

[rhoninl]

Summary

Templated fields can now carry a per-variable fallback: {{ key.path | default text }}. When the key resolves to null/undefined, the literal text is substituted instead of an empty string. Each placeholder gets its own independent default.

Behavior

• Missing → default. {{ a.b | default fallback }} resolves to fallback when a.b is null/undefined.
• No warning when a default is present. Since the author explicitly handled absence, no template_warning is recorded — and a defaulted ref on a critical config field (e.g. agent prompt) no longer fails the node onto its error branch.
• null/undefined only count as missing. An explicit empty-string value is a real value and is not replaced by the default — consistent with the existing warning condition.
• Default value is literal text, trimmed of outer whitespace, may contain internal spaces, cannot contain }. default must be followed by whitespace.

Changes

• lib/shared/template-refs.ts — TEMPLATE_REF_RE extended to capture an optional | default … group; it is now the single shared matcher, imported by the runtime resolver so the lint pass and resolver can't drift. lintField suppresses the "may legitimately be absent" warnings (out-of-scope / missing-global / missing-input) when a default is present, but still flags genuine typos (unknown / missing-field / self-ref) via STRUCTURAL_LINT_REASONS.
• lib/server/templating.ts — resolve() substitutes the default on a missing key and emits no warning when a default is present.
• app/components/TemplateField.tsx — findTemplateSlot clamps the autocomplete slot to the ref name so re-picking a ref preserves an existing | default clause, and stays dormant for an unclosed placeholder where an inserted }} would have nowhere correct to go.
• 23 new test cases across the two *.test.ts files.

Testing

• Full unit suite: 1274 pass / 0 fail.
• Typecheck clean for all changed files.
• Direct functional check of resolve() confirms each scenario (missing→default, present→real value, per-variable defaults, empty default, no-default warning unchanged).

Review

• Design reviewed (tech-lead) — expanded scope to all {{...}} consumers; adopted the narrower lint rule.
• Code reviewed (senior) — caught and fixed a bug where an unclosed {{ ref | default … had no valid spot for an inserted closer.

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