chore(cmd): unify arc update commands and extract internal logic

[jyablonski]

Description

Restructures updates under arc update {system,self,uv}, moves workflows and helpers out of cmd into focused internal packages, and refreshes docs and tests accordingly.

Added

• internal/selfupdate, internal/sysupdate, internal/ghworkflow, internal/gitcleanup, internal/extracmd, internal arcerrs, and thin helpers in internal/ai and internal/notify

Updated

• README / AGENTS for new commands and layout
• Command-tree tests isolate ARC_EXTRA_COMMANDS for deterministic CI

Deleted

• arc self (use arc update self); removed errors.go, command_gate.go, and heavy logic from several cmd files

[coveralls]

Coverage Report for CI Build 25286676802

Coverage decreased (-0.6%) to 55.313%

Details

• Coverage decreased (-0.6%) from the base build.
• Patch coverage: 253 uncovered changes across 12 files (213 of 466 lines covered, 45.71%).
• 4 coverage regressions across 3 files.

Uncovered Changes

Top 10 Files by Coverage Impact	Changed	Covered	%
internal/sysupdate/sysupdate.go	74	0	0.0%
internal/selfupdate/selfupdate.go	132	75	56.82%
internal/ai/usage_cli.go	37	0	0.0%
internal/ghworkflow/dashboard.go	70	36	51.43%
internal/extracmd/extracmd.go	28	7	25.0%
cmd/ai.go	10	0	0.0%
cmd/update.go	11	3	27.27%
internal/gitcleanup/cleanup.go	48	41	85.42%
internal/pacman/kernel.go	23	21	91.3%
cmd/aws.go	1	0	0.0%

Coverage Regressions

4 previously-covered lines in 3 files lost coverage.

File	Lines Losing Coverage	Coverage
cmd/git.go	2	50.0%
cmd/update.go	1	24.0%
internal/ai/presentation/presentation.go	1	21.43%

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Coverage Stats

Relevant Lines:	3755
Covered Lines:	2077
Line Coverage:	55.31%
Coverage Strength:	3.78 hits per line

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💛 - Coveralls