feat(stack): selective image rebuild, Claude plugin tip, demo agent model pin

[bussyjd]

Summary

Three dev-experience improvements that were on the integration branch behind #452 but did not survive the squash merges.

S7 — Selective image rebuild (internal/stack/stack.go, stack_test.go).
OBOL_FORCE_REBUILD_LOCAL_DEV_IMAGES now accepts a comma-separated list of image short names in addition to the existing true/all and false/0/unset modes. Example:

```bash
OBOL_FORCE_REBUILD_LOCAL_DEV_IMAGES=x402-verifier obol stack up
OBOL_FORCE_REBUILD_LOCAL_DEV_IMAGES=serviceoffer-controller,x402-buyer obol stack up
```

Saves ~10 min per iteration when only one image changed. The full set is x402-verifier, serviceoffer-controller, x402-buyer, demo-server, obol-stack-public-storefront (with public-storefront accepted as an alias). Two new unit tests cover both selective rebuild and the alias.

S8 — Claude Code plugin install tip (internal/stack/stack.go).
After obol stack up, if the claude CLI is present but the ObolNetwork/skills marketplace or its plugin is not installed, surface a one-line install hint:

```
Tip: let your Claude Code instance manage your Obol Stack for you.
Add the Obol skills marketplace, then install the plugin:
claude plugin marketplace add ObolNetwork/skills
/plugin install obol@obol
```

Reads ~/.claude/plugins/{known_marketplaces,installed_plugins}.json best-effort and silently no-ops on any error so a malformed Claude config can never block stack up. If the marketplace is already registered, the message switches to just the /plugin install … hint. Fully silent for users without the claude CLI.

S9 — Auto-pin a model on the agent-backed demo (cmd/obol/sell_agent.go).
obol sell agent --demo now resolves the first non-paid/* model from the cluster's LiteLLM config (same source obol model list reads) and writes it into the rendered Agent's spec.model so the controller does not park at ModelUnpinned. Returns a clear "no usable LiteLLM model configured; run obol model setup …" error if the cluster has nothing usable. Also removes a stale "depend on step 2d" caveat that no longer applies now that the controller's in-cluster Hermes provisioning has landed.

Docs updated in CLAUDE.md, .agents/skills/obol-stack-dev/SKILL.md, and .agents/skills/obol-stack-dev/references/dev-environment.md. The Release descriptions / Release Notes blocks added in #456 are preserved untouched.

Test plan

• go build ./...
• go test ./internal/stack/... — including the two new selective-rebuild tests
• On a host with no claude CLI: confirm obol stack up finishes without printing the plugin tip
• On a host with claude installed but no Obol marketplace: confirm the marketplace + plugin install hint appears
• On a host with the plugin already installed: confirm the tip is silent
• OBOL_FORCE_REBUILD_LOCAL_DEV_IMAGES=x402-verifier obol stack up rebuilds only x402-verifier (verify in the build log)
• obol sell agent --demo on a cluster with at least one non-paid/* model: confirm the Agent CR is created with spec.model set