docs: add MAX_JOB_RETRIES, PROXY_URLS, DISABLE_HOSTED_HINTS to .env.example

[MayurK-cmd]

What does this PR do?

Fixes #43
Adds three environment variables to .env.example that the server already reads
but that were missing from the sample config:

• MAX_JOB_RETRIES — max retries per job (default 3), in the Job
  Processing block. Read in server/internal/config/config.go:57.
• PROXY_URLS — comma-separated proxy pool for auto-selection (Thompson
  Sampling), added right below the existing single PROXY_URL fallback.
  Read in server/internal/config/config.go:61.
• DISABLE_HOSTED_HINTS — set to "true"/"1" to suppress hosted
  service tips in error messages. Read in
  server/internal/processor/processor.go:324. Already documented in the
  README.md env table; now also present in the sample file.

No behavior change — these variables already existed and were honored by the
server. This only makes them discoverable for fresh setups that copy
.env.example.

How to test

1. cp .env.example .env
2. Confirm the three new entries are present:
   grep -E "MAX_JOB_RETRIES|PROXY_URLS|DISABLE_HOSTED_HINTS" .env.example
3. Start the server and confirm it boots normally with defaults:
   cd server && TELEMETRY=off go run ./cmd/server
4. (Optional) Set MAX_JOB_RETRIES=9 in .env, restart, and confirm the value
   is honored (the existing config test at
   server/internal/config/config_test.go:112 covers this path).

Checklist

• Tests pass (cd server && go test ./...) — no code changed, config-only
• No breaking changes to existing API endpoints
• Documentation updated — .env.example aligned with config.go,
  processor.go, and README.md