Remove Resource _tag and derive profiles from URI schemes

[schickling]

Problem

Resource identity already lives in uri, but Agent Specs also carried _tag. The two discriminators could disagree and required special legacy handling.

Goal

Make the URI scheme the sole Resource-profile discriminator and reject _tag like any unsupported Resource attribute.

Decisions

• Clean break: no compatibility shim or special _tag diagnostic.
• Preserve exact URI bytes as identity; downstream consumers interpret the scheme.
• Example: resource "work" uri="worktree://dev3/srv/project" relation="used" reason="Current checkout."
• Rejected example: resource "work" _tag="worktree" ....

Verification

• GitHub Nix check: green on exact head 1be318b5d884778bac4dabba97e45558edc4931c
• nix develop -c cargo check --workspace
• nix develop -c cargo test --workspace --no-run
• focused discovery, validate, status-agents, and reconcile tests: 141 passed
• git diff --check

Complexity

Reduces complexity by removing a field and its compatibility paths; no new abstraction or dependency.

Concerns

Breaking syntax change. Consumers must migrate before adopting this st2 revision.

Friction & bottlenecks

No persistent tooling friction or measured bottleneck found in this repository.

Follow-ups

Coordinated consumers are covered by compoundingtech/evals#77 and schickling/dotfiles#1705.

References

Closes #256.

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