refactor: extract shared utilities from duplicated gmail/outlook code

[devin-ai-integration]

Summary

Both gmail-mcp and outlook-mcp duplicated three patterns. This PR extracts them into a new shared/permit0_mcp_shared package.

What was duplicated → what it became:

Pattern	Before	After
Authenticated HTTP client	gmail.py and graph.py each had ~15 identical lines (get token → build headers → httpx.request → error check → return json)	AuthenticatedClient(base_url, error_label, get_token) in permit0_mcp_shared.http
Server creation + transport dispatch	Both server.py had identical FastMCP(name, host=env(…), port=env(…)) + identical main() function	create_server(name) and run_server(server) in permit0_mcp_shared.server
JSON response formatting	json.dumps(obj, indent=2, ensure_ascii=False) repeated ~25× across both servers	to_json(obj) in permit0_mcp_shared.response

Usage after refactoring (gmail.py as example):

from permit0_mcp_shared.http import AuthenticatedClient
from .auth import get_token

_client = AuthenticatedClient(
    base_url="https://gmail.googleapis.com/gmail/v1/users/me",
    error_label="gmail",
    get_token=get_token,
)
call = _client.call  # drop-in replacement for the old `call()` function

Docker build context changed from per-package (gmail-mcp/) to repo root (.) so the shared package is available during COPY. Dockerfiles and docker-compose.yml updated accordingly; the docker build commands in README.md and docs/ are updated too.

Both servers still register exactly 13 tools each — verified by importing and counting.

Link to Devin session: https://app.devin.ai/sessions/88887f3b436449c2923a19a56c1c5c88
Requested by: @fafaisland

[devin-ai-integration]

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