fix: migrate default db path to ~/.local/share/agent-receipts/

[ojongerius]

Summary

Migrates the dashboard's default database path from ~/.agent-receipts/receipts.db to ~/.local/share/agent-receipts/receipts.db to match the daemon, hook, and mcp-proxy (ar#414).

Part of ar#413 — path consolidation across all tools.

Changes

• cmd/dashboard/main.go — replaced defaultDBPath() with XDG-aware version: honours $XDG_DATA_HOME when absolute, falls back to ~/.local/share
• cmd/dashboard/main_test.go — updated existing path assertion; added TestDefaultDBPath_XDGDataHome covering absolute XDG override, relative XDG (ignored), and empty fallback
• README.md, AGENTS.md, CONTRIBUTING.md — updated path references

Test plan

• go test ./... — all 4 packages pass
• go vet ./... — clean

[Copilot]

Pull request overview

Migrates the dashboard's default database path from ~/.agent-receipts/receipts.db to ~/.local/share/agent-receipts/receipts.db, with XDG_DATA_HOME support, aligning with the daemon, hook, and mcp-proxy.

Changes:

• Replaced defaultDBPath() with an XDG-aware implementation (honours absolute $XDG_DATA_HOME, falls back to ~/.local/share).
• Added test coverage for XDG override, relative XDG (ignored), and empty fallback cases.
• Updated documentation references in README.md, AGENTS.md, and CONTRIBUTING.md.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
cmd/dashboard/main.go	Introduces xdgDataHome() helper and updates defaultDBPath() to use XDG path
cmd/dashboard/main_test.go	Updates existing test expectations and adds XDG-specific test cases
README.md	Updates default path reference in usage example
CONTRIBUTING.md	Updates default path references in command tables
AGENTS.md	Updates default path reference in task table

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