fix(detector): align asdf env var on ASDF_DATA_DIR across detector, classifier, docs

[dipto0321]

Summary

The asdf branch of internal/detector/system_node.go's managerManagedRoots was reading $ASDF_DIR to discover the manager's data dir, while internal/detector/asdf.go::asdfDataDir() (the function the detector's actual Install/ListInstalled paths shell out against) reads $ASDF_DATA_DIR — and docs/managers.md documented $ASDF_DIR in the supported-manager table. The three were inconsistent.

A user with ASDF_DIR=/path/to/asdf-source (the source checkout from a git-clone install) would have nodeup classify the node binary under that source dir as manager-managed even though the manager's actual install location was elsewhere (typically ~/.asdf). Conversely, a user with the canonical ASDF_DATA_DIR override would have it silently ignored by the path classifier, leaving their manager install to be reported as unknown in nodeup check and triggering the upgrade-time warning.

Unify all three on $ASDF_DATA_DIR, matching the variable asdfDataDir() already used and the official asdf docs (the source-checkout variable is $ASDF_DIR; the data-dir override is $ASDF_DATA_DIR). Update system_node_test.go's "asdf env wins" case to drive the new variable, and detector.go's package doc comment to match.

Closes #50.

Linked issues

• Closes fix(detector): asdf uses ASDF_DIR vs ASDF_DATA_DIR inconsistently between docs, detector, and system-node classifier #50

Type of change

• fix — bug fix (PATCH bump)

Checklist

• Title follows Conventional Commits (fix(scope): subject)
• I ran make ci locally and it passes (fmt, vet, test pass locally; make lint fails on the v2→v1 schema mismatch tracked in build(lint): .golangci.yml v1 schema incompatible with golangci-lint v2 (brew installs v2, make lint fails to even parse config) #62 — pre-existing on main, independent of this fix; CI runs golangci-lint v1.64.8 per .github/workflows/ci.yml and parses the config fine)
• I added or updated tests for the change (updated system_node_test.go's "asdf env wins" case to use ASDF_DATA_DIR instead of ASDF_DIR; existing asdf_test.go::TestASDF_Detect_HonorsASDF_DATA_DIR and the ASDF_DATA_DIR data-dir resolution tests already cover the rest of the contract)
• I updated relevant docs (README, docs/, inline godoc)
• No new linter warnings
• If breaking: I documented the migration path in the PR body and updated CHANGELOG.md (N/A — no user-visible flag changes; only a previously-ignored env var now works and a previously-honored env var on the wrong directory is no longer honored)

Scope notes

• internal/detector/detector.go had a package-doc comment listing the supported env vars including ASDF_DIR. Updated to ASDF_DATA_DIR.
• The asdf-asdf inconsistency is purely a "wrong env var name" bug, not a parsing or matching issue. Single-token replacement in each of three places.
• I deliberately did NOT support $ASDF_DIR as an additional fallback (some installations use it as an alias for the data dir). The official asdf docs distinguish the two cleanly: $ASDF_DIR is the source checkout, $ASDF_DATA_DIR is the data dir. If a user is using $ASDF_DIR to mean "data dir" they should switch to $ASDF_DATA_DIR — the same variable the rest of their tooling already uses.
• The fix is intentionally scoped to the asdf path on its own; no other manager has this dual-env-var confusion in the codebase.

Out of scope (separate issues)

• --cleanup / --yes precedence conflict — bug(cli): --yes silently overrides --no-cleanup, auto-deletes with zero confirmation #57.
• Path traversal in packages restore/diff via unsanitized manager name — fix(packages): path traversal via unsanitized manager name in packages restore/diff #51.
• Current() failure leaves active Node unprotected from cleanup — bug(cli): Current() failure leaves the active Node version unprotected from cleanup deletion #58.
• n manager's Current() relies on unverified n current subcommand — fix(detector): n manager's Current() relies on unverified 'n current' subcommand #59.

Screenshots / output

User-visible behavior change:

• Before: User with ASDF_DIR=/path/to/source set in their env (typical for git-clone installs) sees nodeup check print a warning that their node binary isn't manager-managed even though asdf fully owns it. Conversely, a user with ASDF_DATA_DIR=/custom sees the classifier claim their node is "manager" only when it happens to land under ~/.asdf (the fallback path).
• After: ASDF_DATA_DIR is the single source of truth: setting it points the classifier at the actual data dir, regardless of what $ASDF_DIR happens to contain.

Verification path: go test -race ./internal/detector/... -run TestSystemNode_ManagerManagedRoots -v (the test table now exercises ASDF_DATA_DIR directly).

[cocogitto-bot]

✔️ 9707118 - Conventional commits check succeeded.