fix: routed-agent context management not respecting history, summarization, compression, and seahorse bootstrap

[j-v]

📝 Description

Bug: I set up dispatch rules to route an agent to a specific discord channel, and noticed it wasn't remembering anything from previous messages, and auto-compaction never triggered regardless of the number of messages or tokens used in the session for that agent & channel.

This PR fixes routed-agent context management so that per-agent context (history, summarization, compression, and seahorse bootstrap) operates on the owning agent's session store instead of the default agent's.

For sessions owned by a routed (non-default) agent, three code paths incorrectly used registry.GetDefaultAgent():

• context_legacy.go — Assemble: read history/summary from the default agent's store, so routed agents always saw empty history (the bug reported in [BUG] /clear and session auto-compression don't work in chats routed to non-default agent via dispatch rules #3301).
• context_legacy.go — maybeSummarize / forceCompression: summarization and overflow compression ran against the default agent's store, leaving the routed agent's context unbounded and uncompressed.
• context_seahorse.go — constructor: bootstrapped only the default agent's sessions, so routed agents never got their session metadata initialized.

Fix:

• Assemble, maybeSummarize, and forceCompression now resolve the owning agent via agentForSession(sessionKey) (the same pattern Clear already used), with fallback to the default agent preserved when the session can't be resolved.
• The seahorse context manager constructor now bootstraps all registered agents' stores (registry.ListAgentIDs()), passing the owning store into bootstrapSession.

Tests: 4 repro tests added covering routed-agent Assemble, summarize, overflow-compression, and seahorse bootstrap — each failed against the old code and passes with the fix.

🗣️ Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

🤖 AI Code Generation

• 🤖 Fully AI-generated (100% AI, 0% Human)
• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)
• 👨‍💻 Mostly Human-written (Human lead, AI assisted or none)

🔗 Related Issue

Fixes #3301

📚 Technical Context (Skip for Docs)

• Reference URL: [BUG] /clear and session auto-compression don't work in chats routed to non-default agent via dispatch rules #3301
• Reasoning: Routed agents (sessions owned by a non-default agent via agentForSession) were being serviced by the default agent's context store in three paths: Assemble (history reads), maybeSummarize/forceCompression (compaction writes), and the seahorse constructor (session bootstrap). For any deployment with routed agents this meant empty history on every turn, no summarization/compaction, and uninitialized seahorse session metadata. The fix follows the existing Clear pattern: resolve the owning agent per-session and fall back to the default agent only when unresolvable.

🧪 Test Environment

• Hardware: GitHub Actions runner (CI) / Raspberry Pi 3B aarch64 (runtime deployment)
• OS: ubuntu-latest (CI), Debian 13 trixie (runtime)
• Model/Provider: N/A — backend agent logic, no model-specific behavior
• Channels: N/A — affects core agent context, channel-agnostic (verified on Discord/Telegram gateways)

📸 Evidence (Optional)

Click to view Logs/Screenshots

Full CI run (fork validation before opening this PR) — all green:

--- PASS: TestLegacyAssemble_RoutedAgent
--- PASS: TestLegacyCompact_Summarize_RoutedAgent
--- PASS: TestLegacyCompact_Overflow_RoutedAgent
--- PASS: TestSeahorseBootstrap_RoutedAgent
ok  	github.com/sipeed/picoclaw/pkg/agent	68.569s

Checks: Tests ✅, Linter ✅, Integration ✅, Security ✅, Cross-Compile (linux/arm64) ✅, gitleaks ✅

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly. (N/A — bug fix, no docs change)

[github-actions]

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[j-v]

Still active
Awaiting review