Add service context for filler

.agents/docs/FUNCTIONS.md

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+- **Config template filling** — renders configuration files from templates by substituting environment variable values, typically as a container start-up step.
+  - **Template discovery** — scans a directory recursively for template files with a given extension, or processes a single template file.
+  - **Value substitution** — replaces placeholders with environment variable values, either via lookup functions or direct variable references.
+    - **Array expansion** — splits a comma-separated environment variable into a list and renders one entry per item.
+    - **Required values** — fails rendering when a variable marked as required, or any referenced variable in strict mode, is missing.
+  - **Output handling** — writes the filled file next to the template without its template extension, or overwrites the template in place; optionally deletes the template afterwards.
+  - **Custom delimiters** — accepts alternative placeholder delimiters so templates can coexist with other templating syntaxes.

.agents/docs/INTERACTIONS.yml

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+---
+internal_calls: {}
+external_calls: {}
+listens: {}
+publishes: []

.agents/docs/service-card.yaml

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+---
+name: filler
+kind: platform
+domain: developer-tooling
+description: |
+  Filler is a small command-line utility that renders **configuration files from templates** by substituting **environment variable** values, typically run as a **container start-up step** before an application boots. It can process whole directories or single files, expand comma-separated variables into lists, and fail fast when **required variables** are missing.