refactor: server-side user prompt construction with @pullfrog tag check (#123)

- Move prompt construction logic from action-side to server-side (webhook handler and trigger page)
- Include issue/comment body in USER PROMPT only if @pullfrog was tagged (checked server-side using containsTriggerPhrase)
- Add repoInstructions as separate REPO-LEVEL INSTRUCTIONS section in FULL prompt
- Macro-expand repoInstructions server-side before sending to action
- Trigger page never includes body (manual triggers)
- Remove redundant customInstructions field (now combined into prompt server-side)

files changed:
- action/external.ts: add repoInstructions to WriteablePayload, remove customInstructions
- action/utils/payload.ts: add repoInstructions to JsonPayload schema, remove customInstructions
- action/utils/repoSettings.ts: add repoInstructions to RepoSettings interface
- action/utils/instructions.ts: use payload.prompt directly, add repo section to full prompt, add repo field to ResolvedInstructions
- utils/webhooks/handleWebhook.ts: check @pullfrog tag and include body if tagged, macro-expand repoInstructions
- app/trigger/[owner]/[repo]/[number]/page.tsx: macro-expand repoInstructions (never include body)

utils/repoSettings.ts

@@ -11,6 +11,7 @@ export interface Mode {
 export interface RepoSettings {
   defaultAgent: AgentName | null;
   modes: Mode[];
+  repoInstructions: string;
   web: ToolPermission;
   search: ToolPermission;
   write: ToolPermission;
@@ -49,6 +50,7 @@ export async function fetchRepoSettings(params: {
       return {
         defaultAgent: null,
         modes: [],
+        repoInstructions: "",
         web: "enabled",
         search: "enabled",
         write: "enabled",
@@ -61,6 +63,7 @@ export async function fetchRepoSettings(params: {
       return {
         defaultAgent: null,
         modes: [],
+        repoInstructions: "",
         web: "enabled",
         search: "enabled",
         write: "enabled",
@@ -74,6 +77,7 @@ export async function fetchRepoSettings(params: {
     return {
       defaultAgent: null,
       modes: [],
+      repoInstructions: "",
       web: "enabled",
       search: "enabled",
       write: "enabled",