fix: standardize secret key names to SCREAMING_SNAKE_CASE

Fixes inconsistent variable naming across the codebase by standardizing
all secret keys to use SCREAMING_SNAKE_CASE format.

**Changes:**
- homeassistant-url → HOMEASSISTANT_URL
- homeassistant-token → HOMEASSISTANT_TOKEN
- database-uri → DATABASE_URI
- pgtuner-exclude-userids → PGTUNER_EXCLUDE_USERIDS
- github-token → GITHUB_TOKEN

**Files updated:**
- chart/templates/deployment.yaml - Fixed secret key references
- README.md - Updated documentation and examples

**Result:**
All secret keys now follow the established SCREAMING_SNAKE_CASE
convention (GITHUB_TOKEN, VNC_PASSWORD, SSH_AUTHORIZED_KEYS, etc.)
for consistency and clarity.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

chart/templates/deployment.yaml

@@ -179,13 +179,13 @@ spec:
               valueFrom:
                 secretKeyRef:
                   name: {{ include "antigravity.envSecretName" . }}
-                  key: homeassistant-url
+                  key: HOMEASSISTANT_URL
                   optional: true
             - name: HOMEASSISTANT_TOKEN
               valueFrom:
                 secretKeyRef:
                   name: {{ include "antigravity.envSecretName" . }}
-                  key: homeassistant-token
+                  key: HOMEASSISTANT_TOKEN
                   optional: true
           livenessProbe:
             tcpSocket:
@@ -215,7 +215,7 @@ spec:
               valueFrom:
                 secretKeyRef:
                   name: {{ include "antigravity.envSecretName" . }}
-                  key: github-token
+                  key: GITHUB_TOKEN
                   optional: true
           livenessProbe:
             httpGet:
@@ -245,13 +245,13 @@ spec:
               valueFrom:
                 secretKeyRef:
                   name: {{ include "antigravity.envSecretName" . }}
-                  key: database-uri
+                  key: DATABASE_URI
                   optional: true
             - name: PGTUNER_EXCLUDE_USERIDS
               valueFrom:
                 secretKeyRef:
                   name: {{ include "antigravity.envSecretName" . }}
-                  key: pgtuner-exclude-userids
+                  key: PGTUNER_EXCLUDE_USERIDS
                   optional: true
           livenessProbe:
             tcpSocket: