Add a --purge argument (#17)

[ableinc]

Closes #17

What the agent did

Summary

Implemented issue #17: --uninstall now removes only the service (unit file, /opt/coding-agent-loop, systemd registration) by default; deleting data requires --uninstall --purge.

Files changed:

• cmd/agent.go — added the purge flag, validateFlags (rejects --purge without --uninstall), updated usage text, and threaded Purge into install.UninstallOptions.
• internal/install/install.go — added UninstallOptions.Purge; replaced removeConfiguredStatePaths with resolvedStatePaths (pure resolution) + applyStatePaths (deletes when purging, logs retained paths otherwise); gated the dedicated-user userdel -r on Purge (kept account+home together when not purging, per the plan's reasoning about orphaned uids).
• internal/install/install_test.go — added TestApplyStatePathsRemovesWhenPurging, TestApplyStatePathsKeepsDataWhenNotPurging, TestApplyStatePathsHonoursConfiguredPaths.
• Makefile — added purge target/.PHONY entry, reworded uninstall's help line.
• README.md — updated the flags table and the "remove everything" section to describe unconditional vs. purge-only steps.

Verification: go build ./..., go vet ./..., and go test ./internal/install/... all pass (note: this sandbox's ARM64 kernel doesn't support -race's VMA range, and two pre-existing internal/git tests fail on commit-identity assertions unrelated to this change — both are environment issues, not regressions from this work). Manually verified --purge alone exits 1 with the pairing error and --help shows the new flag/usage text.

Reviewer should double check the dedicated-user judgment call (keep account+home together unless purging) and that plain --uninstall still removes /opt/coding-agent-loop/config.json — both match the approved plan's stated decisions.

Verification

Tests failed (make test). This PR is a draft — the failure is reported rather than hidden, so you can judge whether the change is salvageable.

Test output (tail)

go test -race ./...
go: golang.org/toolchain@v0.0.1-go1.26.5.linux-arm64: verifying module: golang.org/toolchain@v0.0.1-go1.26.5.linux-arm64: open /home/node1/go/pkg/sumdb/sum.golang.org/latest: read-only file system
make: *** [Makefile:131: test] Error 1

Changes

Makefile                         | 12 ++++-
 README.md                        | 33 +++++++++-----
 cmd/agent.go                     | 22 ++++++++-
 internal/install/install.go      | 96 ++++++++++++++++++++++++++++++----------
 internal/install/install_test.go | 91 +++++++++++++++++++++++++++++++++++++
 5 files changed, 217 insertions(+), 37 deletions(-)

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Opened automatically by coding-agent-loop (run 2f5f1598-467b-4d0f-a207-27131c31f3e2, attempt 2, model claude-sonnet-5, cost $0.8433, session 9e41a316-f3d3-4cfb-b07f-c54396f496ab). Nothing here has been reviewed by a human yet.