[codex] Add history source plugin imports

[luca-ctx]

Summary

Adds local history-source plugin support for custom agent histories on top of the ctx-history-jsonl-v1 import format.

This PR now includes both halves of the intended product shape:

• explicit plugin imports via ctx import --history-source, --plugin, --history-source-manifest, and ctx import --all;
• search-time incremental refresh for installed plugin sources that are both enabled: true and refresh: auto.

A plugin is a local manifest plus a command-line exporter. ctx passes the previous cursor through the runtime environment, reads ctx-history-jsonl-v1 JSONL from stdout, validates source identity/machine identity/schema, imports/indexes the records, and stores the updated cursor.

Search Refresh Behavior

ctx search keeps the existing refresh contract:

• --refresh auto best-effort refreshes native sources and enabled auto plugin sources, then serves existing results if refresh fails;
• --refresh strict fails if native/plugin refresh fails;
• --refresh off never runs provider imports or plugin commands;
• provider-filtered native searches such as --provider codex do not execute custom plugin commands;
• --provider custom can refresh enabled auto plugin sources.

Plugin execution remains local-command based, not a plugin store or in-process ABI. Third parties can ship their own bridge CLI/script, and users can install it into ctx's plugin discovery paths.

Guardrails

• Plugin commands are argv arrays, not shell strings.
• Plugin execution uses a limited inherited environment plus ctx-specific variables.
• Cursor files are private temporary files and are removed after execution.
• stdout is capped at 64 MiB and stderr at 256 KiB per run.
• Timeout now covers child execution plus stdout/stderr pipe draining, including helpers that inherit plugin pipes.
• ctx setup and ctx sources do not execute plugin commands.

Docs

Updated docs describe the custom history source model, the manifest + exporter contract, search-time refresh behavior, and adapter shapes for DorkOS, OpenClaw, Hermes, and NanoClaw.

Validation

• cargo fmt --all --check
• cargo check -p ctx
• cargo test -p ctx --test cli
• cargo test -p ctx-history-capture
• cargo test -p ctx history_source_plugins
• cargo test -p ctx --test cli search_refresh_strict_times_out_when_plugin_helper_keeps_stdout_open
• git diff --check

[luca-ctx]

Closing per maintainer request. Current mainline work has moved past this draft plugin import approach.

[luca-ctx]

Closing as residual/superseded by the 0.15 native provider release path. The shipped stable work went through PRs #21 and #22; plugin architecture can be reopened from a fresh branch/spec if we decide to pick it back up.