feat(cmd): add away mode (vacation) command

[omarshahine]

Depends on #24 — this PR is rebased on top of the auth/gzip/retry fix. Merge #24 first.

Summary

• Adds eightctl away on and eightctl away off commands
• Supports --both flag to toggle both sides of the pod at once
• Adds doURL() to support the separate app API host (app-api.8slp.net)

Details

Away mode tells the pod to stop heating/cooling for a given side, useful when traveling. The API lives on app-api.8slp.net (separate from client-api.8slp.net), which required adding doURL() for absolute-URL requests.

API contract (reverse-engineered from Eight Sleep Home Assistant integration):

PUT https://app-api.8slp.net/v1/users/{userId}/away-mode

# Activate:  {"awayPeriod": {"start": "2024-03-12T08:15:30.123Z"}}
# Deactivate: {"awayPeriod": {"end": "2024-03-12T08:15:30.123Z"}}

Timestamps are set 24h in the past to trigger immediately (same pattern as pyEight and the official app).

Usage

eightctl away on               # your side only
eightctl away on --both        # both sides
eightctl away off --both       # resume both sides
eightctl away on --user-id ID  # specific side

Changes

File	Change
internal/client/eightsleep.go	Add appAPIBaseURL (var, testable), refactor do()→doURL()→doURLRetry(), add SetAwayMode()
internal/client/device.go	Add DeviceSides type and Sides() method
internal/client/eightsleep_test.go	Tests for SetAwayMode (activate/deactivate payloads) and DeviceSides
internal/cmd/away.go	New away command with on/off subcommands, --both flag, --quiet support
internal/cmd/root.go	Register awayCmd

Changes since original submission

• Rebased on fix(client): use form-urlencoded for OAuth token endpoint #24 (auth/gzip/retry fixes, CI fix)
• doURL now uses bounded retries via doURLRetry (inherits the retry cap from fix(client): use form-urlencoded for OAuth token endpoint #24)
• appAPIBaseURL changed from const to var for testability
• Added --quiet support (consistent with other commands)
• Added unit tests for SetAwayMode and Device().Sides()