feat(marketplace): support project-level marketplaces with override warning

[christso]

Closes #223

Summary

• Add --scope <user|project> to plugin marketplace add (default: user) — writes to .allagents/marketplaces.json in workspace root when project
• Add --scope <user|project> to plugin marketplace list — filters by scope; omitting shows both scopes merged
• Add --scope <user|project> to plugin marketplace remove — removes from specified scope; omitting removes from both
• Runtime plugin resolution (plugin@marketplace) uses merged registries with project-over-user precedence
• Override warning emitted when project marketplace shadows a user marketplace of the same name
• marketplace browse and marketplace update also support project-level marketplaces

E2E Test Plan

# Setup
mkdir -p /tmp/aa-e2e/.allagents
echo "plugins: []" > /tmp/aa-e2e/.allagents/workspace.yaml
mkdir -p /tmp/aa-e2e-mp/plugins/test-plugin
echo '{}' > /tmp/aa-e2e-mp/plugins/test-plugin/plugin.json
cd /tmp/aa-e2e

# Test add --scope project
allagents plugin marketplace add /tmp/aa-e2e-mp --scope project
cat .allagents/marketplaces.json  # verify project registry created

# Test add with default scope (user)
allagents plugin marketplace add /tmp/aa-e2e-mp

# Test list (no flag) — shows both scopes merged + override warning
allagents plugin marketplace list

# Test list --scope project — only project entries
allagents plugin marketplace list --scope project

# Test list --scope user — only user entries
allagents plugin marketplace list --scope user

# Test browse — finds plugins from project marketplace
allagents plugin marketplace browse aa-e2e-mp

# Test remove --scope project — user entry survives
allagents plugin marketplace remove aa-e2e-mp --scope project
allagents plugin marketplace list  # shows user only

# Test re-add project, remove --scope user — project entry survives
allagents plugin marketplace add /tmp/aa-e2e-mp --scope project
allagents plugin marketplace remove aa-e2e-mp --scope user
allagents plugin marketplace list  # shows project only

# Test remove (no flag) — removes from both scopes
allagents plugin marketplace add /tmp/aa-e2e-mp --scope user
allagents plugin marketplace remove aa-e2e-mp
allagents plugin marketplace list  # shows nothing

# Test add --scope project without workspace — should error
cd /tmp && allagents plugin marketplace add /tmp/aa-e2e-mp --scope project
# Expected: "Error: No workspace found in current directory."

# Cleanup
rm -rf /tmp/aa-e2e /tmp/aa-e2e-mp

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[christso]

E2E Manual Test Results ✅

All 10 scenarios pass. Build, then run from a temp workspace:

# Setup
mkdir -p /tmp/aa-e2e-223/.allagents
echo "plugins: []" > /tmp/aa-e2e-223/.allagents/workspace.yaml
mkdir -p /tmp/aa-e2e-223-mp/plugins/hello-plugin
echo '{"name":"hello-plugin"}' > /tmp/aa-e2e-223-mp/plugins/hello-plugin/plugin.json
cd /tmp/aa-e2e-223

# Test 1: add --scope project → creates .allagents/marketplaces.json in workspace
allagents plugin marketplace add /tmp/aa-e2e-223-mp --scope project
cat .allagents/marketplaces.json  # verify project registry

# Test 2: add with default scope (user)
allagents plugin marketplace add /tmp/aa-e2e-223-mp

# Test 3: list (no flag) → shows both scopes merged + override warning
allagents plugin marketplace list

# Test 4: list --scope project → only project entries
allagents plugin marketplace list --scope project

# Test 5: list --scope user → only user entries
allagents plugin marketplace list --scope user

# Test 6: browse → finds plugins from project marketplace
allagents plugin marketplace browse aa-e2e-223-mp

# Test 7: remove --scope project → user entry survives
allagents plugin marketplace remove aa-e2e-223-mp --scope project
allagents plugin marketplace list  # shows user only

# Test 8: re-add project, remove --scope user → project entry survives
allagents plugin marketplace add /tmp/aa-e2e-223-mp --scope project
allagents plugin marketplace remove aa-e2e-223-mp --scope user
allagents plugin marketplace list  # shows project only

# Test 9: re-add user, remove (no flag) → removes from both
allagents plugin marketplace add /tmp/aa-e2e-223-mp --scope user
allagents plugin marketplace remove aa-e2e-223-mp
allagents plugin marketplace list  # shows nothing

# Test 10: add --scope project without workspace → error
cd /tmp && allagents plugin marketplace add /tmp/aa-e2e-223-mp --scope project
# Expected: "Error: No workspace found in current directory."

# Cleanup
rm -rf /tmp/aa-e2e-223 /tmp/aa-e2e-223-mp

Scope defaults

Command	--scope omitted	--scope user	--scope project
add	writes to user	writes to user	writes to project
list	shows both	user only	project only
remove	removes from both	user only	project only

Results

Test	Scenario	Result
1	add --scope project creates project registry	✅
2	add (no scope) writes to user registry	✅
3	list (no flag) shows all scopes + override warning	✅
4	list --scope project shows only project entries	✅
5	list --scope user shows only user entries	✅
6	browse finds plugins from project marketplace	✅
7	remove --scope project removes only project entry	✅
8	remove --scope user removes only user entry	✅
9	remove (no flag) removes from both scopes	✅
10	add --scope project without workspace errors cleanly	✅

Unit tests: 926 pass, 0 fail (930 total, 4 skipped)