feat(go): migrate to go 1.26

[garrettladley]

• bump go.mod to go 1.26
• replace pointer-only temporaries with new(expr) in mcp response mapping
• use new(expr) for repository scoreJSON, pagination cursor, and rate-limit reason pointers
• closes feat(go): update to 1.26 #156

[codecov]

Codecov Report

❌ Patch coverage is 28.39506% with 58 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/mcpserver/server.go	32.43%	25 Missing ⚠️
internal/tui/components/chart/cache.go	0.00%	10 Missing ⚠️
internal/server/handler/sse.go	0.00%	5 Missing ⚠️
internal/tui/page/dashboard/sleep.go	0.00%	3 Missing ⚠️
internal/repository/cycles.go	0.00%	2 Missing ⚠️
internal/repository/sleeps.go	0.00%	2 Missing ⚠️
internal/repository/workouts.go	0.00%	2 Missing ⚠️
internal/storage/whoop_redis.go	0.00%	2 Missing ⚠️
internal/xslog/groups.go	0.00%	2 Missing ⚠️
internal/mcpserver/connect.go	66.66%	1 Missing ⚠️
... and 4 more

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